Inga Siebert In the context of ANCOVA (Analysis of Covariance), controlling for change within the control group can be achieved by including the pre-intervention scores as a covariate. This approach allows you to account for any pre-existing differences between the groups and assess the intervention effect more accurately.

Typically, ANCOVA involves including pre-intervention scores as a covariate alongside the group variable (intervention vs. control) to adjust for baseline differences. By including the pre-intervention scores as a covariate, you are essentially controlling for the initial level of the outcome variable before the intervention took place.

To address your concern about checking for change within the control group specifically, you can compare the pre- and post-intervention scores separately for the control group. This analysis will allow you to evaluate whether any significant changes occurred within the control group over time. You can use paired t-tests or a similar statistical test to compare the pre- and post-intervention scores within the control group.

If you find significant changes within the control group, it suggests that factors other than the intervention may have influenced the outcome. This finding would warrant further investigation and might require additional adjustments in your analysis or interpretation of the results.

Remember that in ANCOVA, the primary focus is on comparing the intervention groups while controlling for pre-intervention differences. However, examining and reporting any observed changes within the control group can provide valuable insights into the overall dynamics of the study.

It is important to note that the appropriateness of the ANCOVA assumptions should also be assessed, such as the assumptions of linearity, homogeneity of regression slopes, and normality of residuals. These assumptions ensure the validity and reliability of the ANCOVA results.

In summary, to control for change within the control group in ANCOVA, include the pre-intervention scores as a covariate and assess any significant changes within the control group separately. This approach helps in evaluating the specific effects of the intervention while considering potential changes within the control group.

I hope this explanation clarifies your question. Should you have any further inquiries, please feel free to ask.