Nitrate Levels in the Rock Creek Park Watershed, Washington DC, 1: Measures of Central Tendency

Module by: Mark C. Rains and Len Vacher, University of South Florida

Marian Norris, National Parks Service, Center for Urban Ecology

Cover Page by: Len Vacher and Denise Davis, University of South Florida

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


This Spreadsheets Across the Curriculum activity is the first of a two-part series using water-quality data from Rock Creek Park (Washington DC) to illustrate elementary statistical measures of exploratory data analysis. The environmental focus of the module is nutrient limitation in a stream in an urbanized setting; the mathematics focus is the three measures of central tendency (mean, median and mode). The nutrient data (Jan 2006-2007) span 201 rows of a spreadsheet and are grouped by month. After downloading the spreadsheet, students develop a graph that plots all the nitrate measurements month by month, together with the monthly range lines, means and medians. The data set illustrates how mode, median, and mean fall out in a positively skewed distribution.

This material is based upon work supported by the National Science Foundation under Grant Number NSF DUE-0836566. 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.

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Learning Goals

SSACgnp.TD367.MCR1.5-Slide 8

Students will:

  • Work with nitrate (main part of module) and total phosphorus (end of module assignment) data from 11 stations in the Rock Creek watershed for 2006 and 2007 (201-row spreadsheet, with data grouped by month).
  • Develop a graph that plots the measured values month by month in order to see the range each month, along with the run of monthly means and medians.
  • Explore the meaning of mean, median and mode from a histogram of the positively skewed total data set.
  • Read introductory slides on nutrients, eutrophication, and Washington DC's Rock Creek Park, one of the largest urban parks in the world.

In the process the students will:

  • Get experience with the three main measures of central tendency: mean, median, and mode.
  • See how measures of central tendency are positioned in a skewed distribution.
  • Gain experience producing a moderately complicated scatter plot to show a run of monthly data and their summary monthly statistics.
  • See an example of water-quality monitoring in an urban park.

Context for Use

SSACgnp.TD367.MCR1.5-Slide 13

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 gen-ed quantitative literacy dimension.

Description and Teaching Materials

SSACgnp.TD367.MCR1.5-Slide 15

The module is a PowerPoint presentation with embedded spreadsheets. Click on the link below to download a copy of the module.

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 end-of-module 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 ( by filling out and submitting the Instructor Module Request Form.

Teaching Notes and Tips

The module is constructed to be a stand-alone resource. It can be used as a homework assignment, lab activity, or as the basis of an interactive classroom activity. It was used as an out-of-class 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 this module to be one of the more challenging of the collection, but well within their range of expectations for level of difficulty. It has not been implemented yet in the introductory-level Geology of National Parks course.


There is a slide at the end of the presentation that contains end-of-module questions. The end-of-module 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 end-of-module questions are at the end of the instructor version of the module.

References and Resources

Carruthers, T., S. Carter, L.N. Florkowski, J. Runde, and B. Dennison. 2009. Rock Creek Park Natural Resource Condition Assessment. Natural Resource Report NPS/NCRN/NRR-2009/109. Natural Resource Program Center, Fort Collins, CO.

US National Park Service (NPS)

Rock Creek Park - Biological Informatics Program - Vegetation Characterization Program