Ofthe Variance Research Articles

Usos problemáticos de Internet y depresión en adolescentes: Meta-análisis

Widespread use of the Internet in 21st century society is not risk-free. This paper studies the comorbidity of some problematic uses of Internet with depression in order to assess their correlation. With that aim, a meta-analysis of 19 samples obtained from 13 different studies (n=33,458) was carried out. The subjects of these studies are adolescents between the ages of 12 and 18 years (µ=15.68) from different cultures and continents (Europe, Euro-Asia, America and Asia). The effect size obtained from the use of a random-effects model (r=0.3, p<0.000) is significant, moderate and positive, thus confirming the relation between pathologic uses of the Internet and depression. Moreover, meta-regression test results showed that 9% of the variance (R2=0.09) is associated with the male gender, while age and culture are not significant variables. The variability rate of the studies is high (I2=87.085%), as a consequence of heterogeneity rather than publication bias, as Egger’s regression test shows (1-tailed p-value=0.25; 2-tailed p-value=0.50, and ?=1.57). Therefore, the need for specific interventions in secondary education dealing with this issue is evident to ensure that it does not extend into adult life. El uso extendido de Internet en la sociedad del siglo XXI no está exento de riesgos. El presente trabajo estudia la comorbilidad entre los usos problemáticos de Internet y la depresión con la finalidad de determinar la existencia de relación entre ambas. Para ello se ha llevado a cabo un meta-análisis, que cuenta con 19 muestras recogidas en 13 investigaciones (n=33.748), con adolescentes de entre 12 y 18 años (µ=15,68) de diferentes culturas y continentes (Europa, Euro-Asia, América y Asia). El tamaño del efecto obtenido a partir de un modelo de efectos aleatorios (r=0,3, p<0,000) es significativo, positivo y moderado confirmando la existencia de relación entre usos patológicos de Internet y depresión. Por otra parte, la meta-regresión mostró que el 9% (R2=0,09) de la varianza es explicada por el sexo masculino, mientras que la edad y la cultura no son variables significativas. La variabilidad de los estudios es elevada (I2=87,085%) siendo fruto de la heterogeneidad y no del sesgo de publicación, tal y como indicó la prueba de regresión de Egger (p-value 1 tailed=0,25; p-value 2 tailed=0,50 y ?=1,57). Se advierte la necesidad de comenzar a generar proyectos de intervención en Educación Secundaria que traten esta problemática con el objetivo de que no se extienda a la vida adulta.

Predictors of Social Physique Anxiety among Recreational Bodybuilders

This study aimed to determine the contribution of gender, age, intensity of exercise, perfectionism, perfectionistic self-presentation and eating attitudes in the explanation dimensions of social physique anxiety (SPA) in a sample of 345 male and female gym exercisers. The results of this study indicate that the implemented model explains 21.3 % (p < .001) of the variance in Dissatisfaction with appearance and 48.4 % (p < .001) of the variance in Worry about appearance. Significant predictors of Dissatisfaction with appearance are intensity of exercise, discrepancy and eating attitudes, while significant predictors of Worry about appearance are gender, intensity of exercise, discrepancy, avoiding displays of imperfection and eating attitudes. In other words, recreational levels of exercise, negative perfectionism and negative attitudes towards food are strong predictors of social physique anxiety, while the tendency to hide our imperfections and female gender are only relevant when it comes to worry about appearance.

The Association of Coordination with Physical Activity Levels of Older Adults

Aim: To examine the association between coordination ability and self-reported physical activity among communitydwelling older adults. Methods: We conducted a cross-sectional study of 77 adults (81.51 ± 5.46 years) using motion capture and a gait walkway to assess rhythmic interlimb ankle, shoulder, and gait coordination. Physical activity was assessed using the Physical Activity Scale for the Elderly (PASE). We conducted multivariable linear regression modeling using backward elimination with age, gender, body mass index, Mini-Mental State Exam score, number of chronic conditions, falls, Short Physical Performance Battery (SPPB) score, and interlimb ankle, shoulder, and gait coordination as predictors, and PASE score as the outcome. Results: Gender and SPPB score accounted for 19.4% and the three coordination measures an additional 10%, of the variance in PASE score. Conclusion: The results showed that ankle, shoulder, and gait coordination contribute to self-reported physical activity levels among older adults, even after accounting for SPPB score.

Calculating uncertainty for the RICE ice core continuous flow analysis water isotope record

Abstract. We describe a systematic approach to the calibration and uncertainty estimation of a high-resolution continuous flow analysis (CFA) water isotope (δ2H, δ18O) record from the Roosevelt Island Climate Evolution (RICE) Antarctic ice core. Our method establishes robust uncertainty estimates for CFA δ2H and δ18O measurements, comparable to those reported for discrete sample δ2H and δ18O analysis. Data were calibrated using a time-weighted two-point linear calibration with two standards measured both before and after continuously melting 3 or 4 m of ice core. The error at each data point was calculated as the quadrature sum of three factors: Allan variance error, scatter over our averaging interval (error of the variance) and calibration error (error of the mean). Final mean total uncertainty for the entire record is δ2H=0.74 ‰ and δ18O=0.21 ‰. Uncertainties vary through the data set and were exacerbated by a range of factors, which typically could not be isolated due to the requirements of the multi-instrument CFA campaign. These factors likely occurred in combination and included ice quality, ice breaks, upstream equipment failure, contamination with drill fluid and leaks or valve degradation. We demonstrate that our methodology for documenting uncertainty was effective across periods of uneven system performance and delivered a significant achievement in the precision of high-resolution CFA water isotope measurements.

Factorial Validity of Teacher and School Counselor Support Scales in Latina/o Students

In the current study, we investigated the psychometric properties of two meaningful measures of support from teachers and school counselors among Latina/o students. Examining factorial stability with diverse populations is important to make sure that measures provide valid information about constructs of teacher or counselor support. While researchers have provided evidence of reliability (i.e., consistency), no study has examined validity (e.g., accuracy) of Teacher Support Scale Revised (TSSR) with Latina/o students. Confirmatory and exploratory factor analyses (CFA and EFA) methods were used to evaluate structural validity of the TSSR and School Counselor Support Scale (SCSS). Two different models of the TSSR were analyzed. Findings from multiple CFAs showed that five-factor 22-item of the TSSR had an acceptable model fit. A parallel analysis and EFA were run to determine the factorial structure of the SCSS. Findings indicated that the SCSS had three factors explaining 68% of the variance in the model. The results and implications for research and practice in Latina/o students were discussed

Improving estimation in genetic models using prior information

Statistical models used to investigate research questions in behavioral genetics often require large amountsof data. This paper introduces some key concepts of Bayesian analysis and illustrates how these methodscan aid model estimation when the data does not provide enough information to reliably answer researchquestions. The use of informative prior distributions is discussed as a method of incorporating informationfrom other sources than the data at hand. The procedure is illustrated with an ACE model decomposition ofthe variance of antisocial personality disorder. The data originates from the Norwegian Twin Registry, andincludes adult twins assessed with the Structured Interview for DSM Personality (SIDP-IV). Inclusion ofprior information lead to a shift with respect to conclusions about the presence of shared environmentaleffects compared to a traditional analysis. Small and medium sized studies should consider use of priorinformation to aid estimation of population parameters.

Effects on obese women of the sugar sucrose added to the diet over 28 d: a quasi-randomised, single-blind, controlled trial

To investigate whether obese women can compensate for sucrose added to the diet when it is given blind, rather than gaining weight or exhibiting dysfunctional regulation of intake, in the present study, forty-one healthy obese (BMI 30-35 kg/m²) women (age 20-50 years), not currently dieting, were randomly assigned to consume sucrose (n 20) or aspartame (n 21) drinks over 4 weeks in a parallel single-blind design. Over the 4 weeks, one group consumed 4 × 250 ml sucrose drinks (total 1800 kJ/d) and the other group consumed 4 × 250 ml aspartame drinks. During the baseline week and experimental weeks, body weight and other biometric data were measured and steps per day, food intake using 7 d unweighed food diaries, and mood using ten- or seven-point Likert scales four times a day were recorded. At the end of the experiment, the participants weighed 1·72 (SE 0·47) kg less than the value predicted by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) model; the predicted body weight accounted for 94·3% of the variance in the observed body weight and experimental group accounted for a further 1·1% of the variance in the observed body weight, showing that women consuming sucrose drinks gained significantly less weight than predicted. The reported daily energy intake did not increase significantly, and sucrose supplements significantly reduced the reported voluntary sugar, starch and fat intake compared with aspartame. There were no effects on appetite or mood. Over 4 weeks, as part of everyday eating, sucrose given blind in soft drinks was partially compensated for by obese women, as in previous experiments with healthy and overweight participants.

The linear relationship between growth traits of Sharplanina lambs in extensive farming practices

The fastest phase of growth, observed in young animals, is often assumed to be linear, and linear regressions or ratios between BW gain and time are used to model growth. However, growth curves, due to their flexibility, are likely to be more suitable to describe even early growth. The research was performed in the region of Sharplanina Mountain in the population of the local Sarplanina breed of sheep. The following traits of lambs were considered: BWB, BW30, BW60 and BW90. Statistical analysis was conducted using the Pearson?s correlation and multivariate linear regression model. This involved computing all possible and the best subset regression equation. Each equation was then assessed by its coefficient of determination (R2) and the constant based on the number of variable that used for the prediction. Results showed that weight of lambs from birth to weaning increased by about six times. Specifically lambs achieved an average total gain of 17.66 kg, or 196 g per day. There were a very significant correlation (P<0.01) between BWB and BW30, BW60 and BW90. Likewise, shown a very significant correlation (P<0.01) between BW30-BW60, BW30- BW90 and BW60-BW90. Also shown the coefficient of multiple determinations (R-squared R2) was 0.507 which means that 50.7% of the variance BW90, determined variance of the predictor variables represented in the model. Adjusted coefficient of multiple determination (adjusted R2) is 0.506 which means that 50.6% of the variance BW90, determined variance of the predictor variables that are in the model. Any increase in weight of lambs during the observed period of age is associated with an increase of dependent variable BW90. In particular, any increase in BW30 to 1 kg, is associated with an increase in BW90 to 1.928 kg.

Modeling the Dynamics of Money Income in Nigeria: a Co-Integration Approach

This study examined the relationship between real output, monetary aggregates, price, interest rate and exchange rate usingNigerian data. Analysis of the time series properties of the data revealed that the series are cointegrated which indicated that there is along- run relationship among the variables. We specified an error correction model to analyze the nature of relationship among thevariables. The data for the analysis were sourced mainly from the publications of the Central Bank of Nigeria. The result of theparsimonious model revealed that the error correction term of the broad money model came out with the right sign and significant, implyingthat a shock is rapidly (39%) accounted for in subsequent periods. A one period lag of real exchange rate and price has a negativerelationship with output and significant while M2 and interest rate are positively related to real output and also significant. The result ofthe variance decomposition analysis revealed that the Nigerian data supports the Monetarists’ explanations of business cycles andrecommends however that a combination of both monetary and fiscal policies be explored by the relevant authorities.

Impact of gender and opportunity recognition on attitude to piracy of computer industry products

Software piracy has defied punitive measures, posing threats to jobs of millions of employees and the computer industry. The study proposed anattitudinal approach to the wide-reaching problem and examines the variance explained by gender and opportunity recognition in attitude to piracy of software products. It utilized a correlational design in a survey of two hundred and forty students in an institution of higher learning located in southwestern Nigeria. Results indicate that opportunity recognition and attitudes towardsoftware piracy have a significant positive relationship (â = .18, p < .01). Opportunity recognition accounted for 3.5 percent (p = < .01) ofthe variance in attitudes toward software piracy, indicating that it favourably disposes an individual to software piracy. Contrary to hypothesis, genderwas not significantly related to attitudes toward software piracy (â = .06, n.s.). Practical implications of findings and future research directions are discussed.

The distribution of chironomid larvae and oligochaetes within a stony-bottomed river stretch: the role of substrate and hydraulic characteristics

Combined oligochaete and chironomid data were analyzed to search for main gradients and the separate data sets were analyzed to identify the most important variables determining the structure of the communities. Preferences for particular hydraulic conditions, expressed as Froude number, were calculated for 29 most abundant taxa. The results of Correspondence Analysis (CA) indicated that the main gradient in the data could be explained by hydraulic conditions and, inversely, by the amount of sedimented particulate organic matter (POM). The second CA axis probably reflected the quantity of the available food and space resources. Canonical Correspondence Analysis (CCA) showed the amount of sedimented POM as the most important variable explaining 21 and 24 % of the variance in the oligochaete and chironomid data respectively. Among the other variables, the occurrence of aquatic vegetation, variability of nearbottom current velocity and substrate roughness were the most important.

An analytical characterization for an optimal change of Gaussian measures

We consider two Gaussian measures. In the “initial” measure the state variable is Gaussian, with zero drift and time-varying volatility. In the “target measure” the state variable follows an Ornstein-Uhlenbeck process, with a free set of parameters, namely, the time-varying speed of mean reversion. We look for the speed of mean reversion that minimizes the variance of the Radon-Nikodym derivative of the target measure with respect to the initial measure under a constraint on the time integral of the variance of the state variable in the target measure. We show that the optimal speed of mean reversion follows a Riccati equation. This equation can be solved analytically when the volatility curve takes specific shapes. We discuss an application of this result to simulation, which we presented in an earlier article.

Follicle abnormalities and fibre shedding in Merino weaners fed different levels of nutrition

A study was conducted to examine the relationship between fibre shedding and staple strength in Merino weaners genetically different in staple strength and fed different levels of nutrition. Fibre shedding at the point of break along the staple was estimated using 3 different techniques: (i) a subjective scoring system of wool follicle activity, based on their morphology in transverse skin sections; (ii) the number of fibres with club-ends after differential staining to identify remnants of the shed-follicle bulb; and (iii) changes in the number of fibres in the cross-section along individual staples. Irrespective of the technique used, the estimated proportion of shed fibres did not differ significantly between the sheep bred for sound and tender wool, but increased significantly (P &lt; 0·05) in response to adverse nutritional conditions. Across all treatments, there was a significant (r2 = 0·63; P &lt; 0·001) correlation between the proportion of shutdown follicles and the percentage decrease in the number of fibres in the staple cross-section, although the average difference between the techniques was 11·5%. Both techniques indicated that, on average, about 30% of the total follicle population became inactive and shed their fibre under the most adverse nutritional conditions, and that this was as high as 50–60% for some individual sheep. Neither of these techniques was closely correlated to the proportion of fibres with club-ends (r2 = 0·15 and 0·20, respectively; P &lt; 0·01). The proportion of shutdown follicles and the percentage decrease in the number of fibres in the staple cross-section explained 54% and 52% of the variance in staple strength, respectively, compared with only 19% explained by the percentage of fibres with club-ends at the point of break. However, as fibre shedding failed to remove any variance in staple strength additional to that already attributed to along- and between-fibre changes in diameter, it is concluded that fibre shedding per se does not contribute significantly to nutritional-induced differences in staple strength.

Relationship between drop vertical jump heights and isokinetic measures utilizing the stretch-shortening cycle

The purpose of this study was to determine how isokinetic eccentric and concentric peak torque/body weight ratios and time to peak torque values relate to drop vertical jump heights in 30 healthy subjects (12 men and 18 women, age = 22.7±2.2 yr, ht =169.6±8.0 cm, wt =67.2f11.9 kg). Drop vertical jumps were performed from a box 50 cm high. Subjects were tested isokinetically on each leg for the motions of knee extension and ankle plantar flexion. Eccentric and concentric peak torque (PT)/body weight ratios along with time (s) to eccentric and concentric peak torque were extracted from the Kin Com II computer. A stepwise multiple regression analysis was conducted to determine the relationship between the eight predictor variables and drop vertical jump height. Knee extension concentric PT/body weight ratio was the most significant predictor of drop vertical jump height, accounting for 23% of-the variance. This finding suggests that the stretch-shortening cycle can be simulated using isokinetic strength measurements and that drop vertical jump height can be attributed in part to concentric knee extension strength.

Preface to 'Role of Reactive Phosphate Rock Fertilisers for Pastures in Australia'

Summary. The dissolution of North Carolina phosphate rock (NCPR) in soil was investigated in a laboratory study using surface soils sampled from 28 permanent pasture sites. The relationships between phosphorus (P) dissolved, P availability and various soil properties were investigated using simple and multiple linear regression and the findings related to the relative effectiveness of NCPR for pasture production at the sites. The extent of dissolution of NCPR was positively correlated to P buffering capacity (r2 = 0.42). Phosphorus buffering capacity and titratable acidity together accounted for 72% of the variance in dissolution. Bicarbonate-extractable P (‘available’ P) generally increased as dissolution increased. However, the increase in available P was consistently lower for soils with higher P buffering capacity. The proportion of dissolved P that was available also decreased with increasing P buffering capacity (r2 = 0.63). Consequently, the increase in available P was highest for soils with very low to low P buffering capacity. This suggests that the effectiveness of NCPR as a fertiliser may be more closely related to the availability of dissolved P, than to the amount of NCPR dissolved in a soil. Consistent with this laboratory finding, the agronomic effectiveness of NCPR relative to superphosphate measured in the field tended to decrease with increasing P buffering capacity. The agronomic effectiveness of NCPR was comparable with superphosphate only at certain sites, and with some noted exceptions, most of these had surface soils with very low to low P buffering capacity. The high relative effectiveness of NCPR at these sites was mostly attributed to the loss of superphosphate by leaching. Since NCPR dissolves much more slowly than superphosphate, only a small amount of the P applied as NCPR would be lost during leaching events. Slow dissolution of the remaining NCPR probably supplied a small amount of dissolved P over an extended period of time, and due to the low P buffering capacity, much of this was available to plants.

The effect of phosphate buffering capacity and other soil properties on North Carolina phosphate rock dissolution, availability of dissolved phosphorus and relative agronomic effectiveness

Summary. The dissolution of North Carolina phosphate rock (NCPR) in soil was investigated in a laboratory study using surface soils sampled from 28 permanent pasture sites. The relationships between phosphorus (P) dissolved, P availability and various soil properties were investigated using simple and multiple linear regression and the findings related to the relative effectiveness of NCPR for pasture production at the sites. The extent of dissolution of NCPR was positively correlated to P buffering capacity (r2 = 0.42). Phosphorus buffering capacity and titratable acidity together accounted for 72% of the variance in dissolution. Bicarbonate-extractable P (‘available’ P) generally increased as dissolution increased. However, the increase in available P was consistently lower for soils with higher P buffering capacity. The proportion of dissolved P that was available also decreased with increasing P buffering capacity (r2 = 0.63). Consequently, the increase in available P was highest for soils with very low to low P buffering capacity. This suggests that the effectiveness of NCPR as a fertiliser may be more closely related to the availability of dissolved P, than to the amount of NCPR dissolved in a soil. Consistent with this laboratory finding, the agronomic effectiveness of NCPR relative to superphosphate measured in the field tended to decrease with increasing P buffering capacity. The agronomic effectiveness of NCPR was comparable with superphosphate only at certain sites, and with some noted exceptions, most of these had surface soils with very low to low P buffering capacity. The high relative effectiveness of NCPR at these sites was mostly attributed to the loss of superphosphate by leaching. Since NCPR dissolves much more slowly than superphosphate, only a small amount of the P applied as NCPR would be lost during leaching events. Slow dissolution of the remaining NCPR probably supplied a small amount of dissolved P over an extended period of time, and due to the low P buffering capacity, much of this was available to plants.

Seasonal changes in the morphology of wool follicles in Finewool and Strongwool Merino strains grazing at different stocking rates in southern Australia

An experiment was conducted to determine the impact of stocking rate and Merino strain on follicle morphology before and after the break of the season in the highly seasonal Mediterranean environment of southern Australia. Groups of Finewool and Strongwool Merino sheep were allocated to 9 stocking rates on mixed legume-grass pastures, and skin biopsy samples were taken at monthly intervals from February to June. A scoring system, based on the morphology of follicles in transverse section, was used to characterise these samples. The proportion of follicles classified as ‘normal’ dropped markedly, and the proportion of follicles which contained no fibre correspondingly increased, after the break of the season in April. On average about 10% of the follicles became inactive but there was considerable variability (range 2-63%) between animals. The proportion of inactive follicles was significantly affected by stocking rate but there was little difference between Merino strains. Maximum follicle inactivity coincided with the period of minimum fibre diameter and minimum liveweight in May, approximately 1 month after the break of the season. The proportion of inactive follicles accounted for 27% and 28% of the variance in staple strength of the Finewool and Strongwool strains, respectively. Minimum fibre diameter accounted for 63% and 61% of the variance in staple strength, and coecient of variation in fibre diameter accounted for 49% and 58% of the staple strength variance, respectively, in the 2 strains. Together, minimum fibre diameter and coecient of variation in fibre diameter accounted for almost 75% of the variance in staple strength in both strains. Addition of a term for the proportion of inactive follicles did not remove any additional variance in staple strength. These results suggest that the follicles of Merino sheep in Mediterranean environments undergo significant morphological changes throughout the year. These changes differ from the normal sequence of events associated with the hair cycle and appear to be associated with the break of the season in autumn. The morphological changes which occur in the follicles are similar to those induced by epidermal growth factor or cortisol, and may reflect a stress response. Our results suggest that nutritional stress is at least partially responsible for the follicular pathology described. Management strategies aimed at reducing the decrease in fibre diameter which occurs in autumn, shearing sheep in autumn to coincide with the minimum fibre diameter, and selection of sheep which have a low coecient of variation of fibre diameter, are likely to be the most effective means of preventing low staple strength in sheep grazing in Mediterranean environments. Nevertheless, the impact of follicle shutdown and changes in follicle morphology on wool characteristics other than staple strength needs to be determined.

Psychological distress in police officers following critical incidents

This study considers the impact of a range of potentially traumatic events on a cohort of police officers. All RUC officers who were involved in critical incidents (July 1993–January 1994) were contacted at 7–10 months post incident. Following their verbal agreement to participate in the survey, officers received a questionnaire booklet requesting further details and subjective evaluation of the index incident. The booklet contained a checklist of PTSD symptoms (MPSS-SR), Beck Depression Inventory (BDI) and a brief measure of support (Crisis Support Inventory - CSI). Analysis of returns indicates that around 5% of the sample would fulfil current criteria for PTSD. In addition 25% of officers reported symptoms consistent with at least mild to moderate depression (BDI). Regression analysis noted that three factors: BDI score, Injury Rating and Present Satisfaction with Support explained 50% ofthe variance in PTSD total. The relationships between PTSD, Depression and Injury are discussed. In addition some limitations of the present study as well as avenues for future research are highlighted.

Patterns of Mammalian Species Richness in the Australian Tropical Rainforests: Are Extinctions during Historical Contractions of the Rainforest the Primary Determinants of Current Regional Patterns in Biodiversity?

Distribution data on the mammals of the wet tropics have been used to analyse biogeographic patterns in assemblage composition and to correlate patterns of species richness with environmental factors such as climate and vegetation. Multivariate analyses suggest five different geographically separated assemblages of rainforest mammals. The most species-rich is found in the central uplands (Atherton Tableland) with a decrease in species richness to the north and south and with decreasing altitude. The most species-rich areas are characterised by large areas of rainforest with a rounder shape (low shape index), high annual rainfall, consistent rainfall in the dry season and a diversity of rainfall regimes within the area. Multiple-regression analysis suggests that the combination of rainforest area and shape explain the most variance (r2 = 0·74) in the patterns of species richness of rainforest mammals. Various measures of habitat diversity are also highly dependent on area, and a similar degree of the variance in species richness (r2 = 0·78) can be explained by using rainforest shape and habitat-diversity variables (rainfall and vegetation diversity) and excluding area. This suggests that the effect of area on the patterns of species richness is primarily due to its positive influence on habitat-heterogeneity factors in the regression. Analysis of the guild structure (number of guilds and the species richness within each guild) indicates that it is the number of species within guilds that most strongly affects patterns of species richness in rainforest, although the number of guilds also has an effect. Most of the variance in species richness can be attributed to three (primarily arboreal) guilds that have previously been shown to be the most extinction- prone species in the wet tropics. These patterns suggest the hypothesis that current patterns of mammalian species richness in wet tropics rainforest are primarily the result of localised extinctions in those areas most affected by Pleistocene contractions of the rainforest. The relative impacts of these contractions on each rainforest block are indexed by current area and shape.

The nature of the influence of speed on adult age differences in cognition.

Two studies were conducted to determine the relations among age, motor speed, perceptual speed, and 3 measures of cognitive performance: study time, decision time, and decision accuracy. Each study involved over 240 adults between I 8 and over 80 years of age who all performed a battery of tests, including computer-administered tests of memory, reasoning, and spatial ability. The results indicated that (a) increased age was associated with lower accuracy as well as with longer study time and decision time and (b) some ofthe relations between age and decision accuracy and between age and decision time appear to be mediated by a slower rate ofexecuting cognitive operations. Considerable evidence now exists indicating that a large proportion ofthe age-related variance in many different cognitive variables is shared with a measure of perceptual speed. Among the most pertinent results are the findings that the age-related diffbrences in various measures of cognitive functioning are greatly reduced when statistical control procedures are used to adjust for differences in perceptual speed. As an illustration of this phenomenon, Thble I in Salthouse (1993b) contains 44 comparisons across a wide range of cognitive variables. Age was associated with a mean of l6.2Vo of the total variance in the variables, but after the variance associated with measures of perceptual speed was held constant, age was associated with only 3.6Vo ofthe variance in the cognitive variables.