Equivalence Test for 2x2 Crossover Design

Effects

  

The Effects table shows the analysis for the following possible effects:

·    Carryover: The carryover statistic measures the effect of one treatment on the next. For example, suppose the reference treatment has a strong effect and the test treatment has a weak effect. If the washout period is not long enough, the residual effects of the reference treatment in Period 1 can cause the effects of the test treatment in Period 2 to appear stronger than they actually are.

·    Period: The period statistic measures the difference between the response in Period 1 and in Period 2. For example, if you measure blood pressure as your response, you might find that the response decreases during Period 2 simply because participants are more acclimated to the testing environment and procedures. This could result in a period effect.

Note

You should not evaluate equivalence if either the carryover effect or the period effect is significant. If either effect is significant, then the equivalence results may be unreliable.

·    Treatment: The treatment statistic measures the difference between the effects of the test treatment and of the reference treatment. In most studies, the treatment effect is the effect of interest.

Example Output

              Effect        SE  DF  T-Value  P-Value         95% CI

Carryover    0.45181   0.64988  15  0.69521    0.498   (-0.93339, 1.8370)

Treatment   -0.32104  0.060641  15  -5.2941    0.000  (-0.45030, -0.19179)

Period     -0.097708  0.060641  15  -1.6112    0.128  (-0.22696, 0.031546)

Interpretation

For the antacid data, the p-value for the carryover effect (0.498) and the p-value for the period effect (0.128) are both greater than 0.05. Thus these effects are not significant at the 0.05 level.

The p-value for the treatment effect (0.000) is less than 0.05. Thus the treatment effect is significant at the 0.05 level. The significant treatment effect indicates that one antacid is better than the other at raising stomach pH. However, a significant treatment effect does not mean that you cannot claim equivalence. The difference between the treatment means may still be within your equivalence limits.