Wading Through the Data Swamp:
Program Evaluation 201

Supplements

The t-Test of Differences Between Means:
Independent Samples t-Test for Posttest Data

Step 7: Complete the sum of squares (SS) calculation.

The sum of squares is the sum of squared deviations from the mean.

The formula is:

formula for completing the sum of squares calculation

The different parts of the formula have been calculated in steps 5 and 6:

  1. The first part of the formula corresponds to step 6: formula for step six where each score for both groups in squared and added together
  2. The numerator of the second part was calculated in step 5: formula showing result of previous number as the numerator over n, to total number of participants
  3. The denominator, n, represents the total number of participants.

For our example, both the participants and comparison group have 50 members. Our next step is to insert all the numbers in the formula.

Sum of squares (SS) calculation for the participant group:

another sum of squares formula for participant group

SS = 558 - (13456/50) = 288.88

Sum of squares (SS) calculation for the comparison group:

formula for completing the sum of squares calculation

SS = 1157 - (30625/50) = 544.5

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