t-test of Fecal Coliform Data

In this activity, you will be performing a t-test on the fecal coliform counts collected from Blackwater Creek in Lynchburg, Virginia. Blackwater Creek is known to have a problem with fecal coliforms due to sewage overflow from an old sewer system that mixes storm drainage and sewage. Hence, when it rains, sewer pipes back up and sewage is dumped into the creek. Several studies are being conducted by students at the Central VA Governor's School for Science and Technology to quantify the extent of the fecal coliform concentration in the creek associated with rainfall.

Fecal coliforms are often found in the mud in the bottom of rivers and streams. When it rains, the increased flow of water disrupts the bottom sediments and fecal coliforms are released into the water.

This study was designed to determine if fecal coliforms reside in the sediments of Blackwater Creek. Water samples were taken in three locations along the creek; Sandusky Park, Blackwater Athletic Area and Hollins Mill. First, a water sample was taken, then the sediments at the bottom of the creek were disrupted with a shovel and a second sample was immediately taken downstream. Samples were collected over a four week period in the fall of 1995. The data represents 12 samples taken before disruption and 12 samples taken after disruption.

Using the data set in Excel, conduct a t-test to determine if there is a significant difference between the two samples.

1. What did the results of the t-test suggest about the effect of disrupting the bottom of the creek?

Eliminate the 4200 and the 1200 readings from the "Before Disruption" data set and the 7300 and the 8400 readings from the "After Disruption" data set and redo the t-test.

2. Imagine that this was the data set you collected. What would these results have suggested about the effect of disrupting the bottom of the creek?
3. Explain the effect of the removal of those data points on the results of the t-test.
4. Why did removing those data points have that effect?

Original work on this document was done by Central Virginia Governor's School student Crystal Peery (Class of '97).

Copyright © 2011 Central Virginia Governor's School for Science and Technology Lynchburg, VA