What type of effect does the horizontal line indicate in a confidence interval?

The horizontal line in a confidence interval typically represents the null hypothesis value, which indicates no effect. In the context of a confidence interval for a treatment effect or a difference between groups, when the confidence interval crosses this horizontal line, it suggests that the effect might not be statistically significant. This means that there is a plausible chance that the true effect could be zero, indicating no significant difference or no effect at all.

For instance, if you are analyzing the effect of a treatment and the confidence interval for the difference in outcomes includes zero, this implies that you cannot confidently conclude that the treatment has an effect, as the range includes the possibility that there is no effect (the null hypothesis). Therefore, this horizontal line serves as a visual representation of the potential for a lack of significant effect in the context of hypothesis testing.