Paired TTest

Earlier this year, biology classes put elodea plants under water in test tubes. They then held them 5 cm from the light or 15 cm from the light to see if the number of bubbles of oxygen changed. You could designate the 5cm away from the light the control group and the 15 cm away from the light the experimental group. The treatment independent variable was moving the plants away from the light by 10 cm. The results of our 5th period class are summarized in the following Excel spreadsheet:

To accomplish a ttest on paired data, you need to follow these steps:

1) Get averages of the two data sets by adding them up and dividing by the number of measurements. When using Excel, you can insert the function =AVERAGE() and let the spreadsheet figure it.

2) Find the difference between the two averages.

3) Insert a "TTEST" Function just under the cell that has the average.

4) "Array 1" is the first set of data. So click the little spreadsheet to the right of "Array 1" and highlight the control data set and hit enter.

5) "Array 2" is the experimental data set 2. Click the spreadsheet to the right of "Array 2", highlight the experimental data, and hit enter.

6) Enter the number "2" into the "Tails" box because there are 2 groups you are comparing. Then enter "1" under "Type" box because this is a "Type 1" Paired t-Test.

7) Hit "OK" and you are finished. Your probability statement is now ready for your analysis. "P=.02" means that there are 2 chances in 100 that the null hypothesis is correct. Since it is less than P=.05 (scientific significance cut off), we found a significant difference between the control plants 5 cm away from the light and the experimentals that were 15 cm away.

If I were writing about this in a paragraph 1 of my "discussion," I would say: "My H1 hypothesis stating the distance from the light would make the oxygen bubbles less, was supported by the data. We found that plants 5 cm from the light averaged 19.5 bubbles/3min while the plants 15cm from the light averaged 3.2 bubbles/3min. with a difference of 16.4 bubbles/3min (p=.02). I can state that increasing the distance from the light decreases the amount of photosynthesis with a 98% confidence level."