You must type this and all other homework assignments. Do not e-mail the assignment to me; turn it in early (at 322 Wolf) for a foreseeable absence, or turn it in late after an unexpected absence from class.
1. Collect some measurement data. You must have one nominal variable with at least four values (such as undergraduate, graduate, post-doc, faculty, or dog, cat, bunny, ferret) and one measurement variable (such as nose width or number of fleas). You must have at least ten observations in each of your four or more categories. Your data set could be a published data set, some data you've collected for your research, or some data you collect for this assignment. If you can't think of anything else, you could collect tree leaves (in at least four categories, such as four different trees) and measure their length.
Try to pick groups where the difference in the measurement variable is not so big that it's obvious the statistical test will be highly significant; for example, if you measure leaves, measure leaves from four or more trees of the same species, not different species with obviously different leaves.
Put your raw data (the individual observations) in a table. For each category, calculate the mean, median, range, standard deviation, standard error of the mean, and 95 percent confidence interval (you may use a spreadsheet, web page or computer program for this). Add these summary numbers to the table.
Keep a copy of your raw data; you'll need it for the next homework assignment.
2. Draw a bar graph, with vertical columns representing the means and thin vertical lines representing the 95 percent confidence intervals. Your graph should have a legend underneath it, explaining what it is.
|
| The number of bird species observed in the Christmas Bird Count at seven locations in Delaware. Data points are the mean number of species for the counts in 2001 through 2006, with 95 percent confidence intervals. |
3. Flip through your favorite scientific journal until you find a graph with vertical bars around the means (commonly called "error bars"), like the ones above. Tell me what the bars represent: standard deviation, standard error, 95% confidence limits, or something else. If the paper doesn't say what the bars are, tell me that (if the information isn't in the figure caption, look in the Methods section and in the captions to other graphs). Also list all of the variables shown on the graph, and say whether each one is a nominal or measurement variable. Give the citation information (authors, year, article title, journal, volume, page numbers). Print or photocopy the page with the graph on it (not the whole article) and attach that to your homework.
4. Look at the data you collected in question 1 and compare the smallest mean to the largest mean, using Student's t-test. Use a spreadsheet, then use one of the web pages linked from the Handbook web page on t-tests. For each, report the t-value, the degrees of freedom, and the P-value. Then do the t-test using SAS. If you get SAS to work, print your SAS program and the output. If you can't get SAS to work after a reasonable length of time, print your SAS program and the error log (if you got that far) or whatever error message you got that stopped you.
Return to the Biological Data Analysis syllabus
Return to John McDonald's home page
This page was last revised August 4, 2009. Its URL is http://udel.edu/~mcdonald/stathw4.html