[ILLUSTRATION OMITTED] People are regularly confronted with environmental and science-related issues presented to them in newspapers, on television, or even in their own doctor's office. Often information they use to inform their decisions on matters of science may be ambiguous and contradictory. This article presents an activity that investigates how students deal with ambiguity in scientific data and whether they use such information to make decisions. The activity connects to Next Generation Science Standards (NGSS Lead States 2013) practices of analyzing and interpreting data (Practice 4); engaging in argument from evidence (Practice 7); and obtaining, evaluating, and communicating information (Practice 8). Environmental science literacy, the capacity [of an individual] to understand and participate in and make decisions through evidence-based discussions of socio-ecological systems, is important, because all citizens should be able to understand and evaluate evidence and make informed decisions using such evidence (Moore 2009, p. 1). Understanding ambiguity in data is a core issue for scientific inquiry, but many people do not comprehend it. Telling general public that data are uncertain may reduce their trust in science and scientists (Frewer et al. 2003) and lead policy makers to avoid or delay making decisions when faced with contradictions in data (Michaels and Monforton 2005). Kolsto (2001) hypothesized that public frustration with socioscientific issues was due to perceived disagreement among scientists and experts; at same time, science in textbooks often appears undisputed and clearcut, which misleads students. To address that misconception, instead of doing cookbook laboratory investigations with a single correct result, students should engage in messy labs that highlight inconclusive and ambiguous results, revealing ambiguity at heart of science (Metz 2005). [FIGURE 1 OMITTED] Ambiguity in data is common. Nearly all data sets contain variation (or uncertainty) that students may see as ambiguity. Data may also seem ambiguous when students doubt trustworthiness of source, when details about experiment seem insufficient, or when scientists disagree about interpretations. Thus, we designed an activity called Ambiguity and Evidence in which students grapple with ambiguous and conflicting data to make science-related decisions. The activity Ambiguity and Evidence provides a context to initiate classroom discussions about ambiguity in scientific data and how students can make decisions using ambiguous information. The purpose is not to advocate that students make a specific decision but rather to demonstrate how evidence (even when ambiguous or incomplete) plays a role in decisions about scientific issues. The scenario In this one-period (about 50 minutes) classroom activity, 22 11th- and 12th-grade students in an Advanced Placement Environmental Studies class were tasked with deciding whether to develop an abandoned grassy school yard area into a concrete parking lot for cars and bikes. The grassy yard was described as connected to a river that flows into drinking water supply (Figure 1). The key science issue was how change from grass to concrete would affect water quality and ecosystem health. We asked students: Before reading additional evidence, would you be likely to support development of abandoned grassy field into a concrete parking lot? Yes or No? and, Why or why not? Reading and using evidence We created two sets of supplemental information for students to read before making a final decision--one biased toward building parking lot (packet 1) and other biased toward leaving grassy field alone (packet 2). Packets were assigned to students based on their initial decision. Students in favor of development were given packet biased toward leaving grassy field as is, and students opposing parking lot were given packet leaning toward developing it. …