Nitrate Levels in the Rock Creek Park Watershed, Washington DC, 2: Variability
Summary
This material is based upon work supported by the National Science Foundation under Grant Number NSF DUE0836566. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Learning Goals
Students will:
 Examine an outlier in the nitrate data they plotted in the first module of this two part series.
 Calculate variance and standard deviation of the nitrate deviation two ways: brute force from the definitions, and the easy way using Excel's builtin function.
 Calculate zscores for the nitrate values
 Do a site calculation using the normal frequency distribution and the normal distribution function to determine the probability of measurements within one, two and three standard deviations of the mean (68, 95, and 99.7%, respectively) for normally distributed data.
 See visually that these data are not normal (the data are positively skewed), but nevertheless calculate that the zscore of the outlier value is impressive (more than 6).
 Consider the effect on the summary statistics (mean, median, mode, range, variance, standard deviation) if the outlier were disregarded
In the process the students will:
 Get familiar with the variance and standard deviation and see some uses for them (expressions of variability; a way of expressing distance from the mean; a way to evaluate an outlier)
 See that range and mean are sensitive to outliers.
 Learn where the magical 68, 95, and 99.7% values come from, and that not all distributions are normal.
Context for Use
This module is designed for potential use in the Geology of National Parks service course at USF. The course is offered as an online course every semester. It includes readings from Parks and Plates, weekly quizzes based on that textbook, and weekly student activities designed to align the course with the University's general education requirements. This module is intended to be one of those activities, with the specific goal of meeting the gened quantitative literacy dimension.
Description and Teaching Materials
The module is a PowerPoint presentation with embedded spreadsheets. Click on the link below to download a copy of the module.

PowerPoint SSACgnp.TD367.LV1.10 (PowerPoint 7.7MB Jun1 13)
Optimal results are achieved with Microsoft Office 2007 or later; the module will function in earlier versions with slight cosmetic compromises. If the embedded spreadsheets are not visible, save the PowerPoint file to disk and open it from there.
The above PowerPoint presentation file is the student version of the module. It includes a template for students to use to complete the spreadsheet(s) and answer the endofmodule questions, and then turn in for grading.
An instructor version is available by request. The instructor version includes the completed spreadsheet. Send your request to Len Vacher (vacher@usf.edu) by filling out and submitting the Instructor Module Request Form.
Teaching Notes and Tips
The module is constructed to be a standalone resource. It can be used as a homework assignment, lab activity, or as the basis of an interactive classroom activity. The Rock Creek 1 and 2 set was used as an outofclass activity in Computational Geology (a QL course for geology majors) in Fall 2010 and Fall 2011 after the students had worked through several other modules, notably the two about Yellowstone geysers involving histograms. In general, the students considered these modules to be among the more challenging of the collection, but within their range of expectations for level of difficulty. They have not been implemented in the introductorylevel Geology of National Parks course.
Assessment
There is a slide at the end of the presentation that contains endofmodule questions. The endofmodule questions can be used to examine student understanding and learning gains from the module. Pre/post test, pre/post test answer key, and answer key for endofmodule questions are at the end of the instructor version of the module.
References and Resources
Rock Creek Park
US National Park Service (NPS)
