Descriptive Statistics

Confidence Interval for Seed Harvester Ant Data

In this activity, you will be calculating the confidence interval for the seed harvester ant data set. The data for the file was collected at the Sierra Nevada Aquatic Research Laboratory (SNARL) in the Great Basin Desert Province. This study investigated the relationship between the size of the ants and the weight in milligrams of each ant. The units for the class sizes are arbitrary, corresponding to ruled increments that can be seen in the microscope with which the sample is measured. Using Excel, determine the mean and the standard deviation for the weights for each class size in the seed harvester ant data set. Then, using these two values for each class size, calculate the confidence intervals for each class size in the seed harvester ant data set. If you choose to use the TI-83 graphing calculator, the mean, standard deviation, and confidence interval values are calculated at the same time.

The data exists in three files; one is in Excel Data format, one is in Text format, and one is in TI-83 Group format. If you want to determine the confidence interval using the TI-83, you will have to import the data into the calculator from the computer after you have downloaded the file.

What is the confidence interval for each size class using 80% confidence? 95% confidence? 99% confidence? Find the confidence interval for each size class using 99% confidence. How do the confidence intervals change if you compare the 80%, 95%, and 99% confidence intervals for the <37 class ant? Do you detect the same change in the other three classes? Can you explain why the range of the confidence interval changes as it does when you go from 80% to 95% to 99%? A fairly large group of ants have a mean weight of 59.4 mg and all belong to the same class: what class would they most likely belong to?

Original work on this document was done by Central Virginia Governor's School students Jonathan Kim, Steven Nash, and Corey Templeton (Class of '05).