I want to use a multiple logistic regression to model the relationship between two experimental groups (test and control) and accuracy of a procedure, controlling for the experience (in years) of the participants.

outcome ~ group + experience

The design I am using is paired in the sense that every participant is tested twice, so there are no differences in baseline characteristics between groups (since they are the same individuals). If I was only testing for differences in time, a paired t-test would suffice, but I need to control for experience, hence a regression model is being built.

Time is measured in seconds until the procedure is completed, and accuracy is defined as completing it within a pre-specified threshold (the outcome is 1 if less than or equal to 6 minutes and 0 otherwise). It is expected that time and experience are negatively correlated or, experience practitioners are expected to take less time to complete the procedure.

I would like to test for interactions in this model, but I don't think it makes much sense to interact the group with experience.

outcome ~ group * experience

I am considering including time in the model and test for interaction with experience.

outcome ~ group + experience*time

Since time is used in the definition of the response of the logistic model I expect it to be significant even with a small sample size. However it seems to me that including time in this model would be circular reasoning.

outcome ~ group*time + experience

Q1: Is this a correct interpretation?

Q2: If I try interactions between time and the group instead, would that tell me that time is modifying the effect attributed to the group?

Q3: Does it make sense to test for interactions between experience and group in this setting?

EDIT: I understand Douglas Altman's point of that, while unnecessary dichotomization of a continuous variable is prevalent in medical research, it leads to loss of estimate precision (at the very least). I was able to make the case for a linear model of time ~ group + experience for this experiment as a secondary endpoint, but the main goal needs to remain being accuracy, which is why the outcome is a dichotomization of time. This practice is prevalent for a reason :)

  • $\begingroup$ Could you include the formula/specification for the different models you are considering? $\endgroup$
    – mkt
    May 11, 2023 at 11:07
  • $\begingroup$ @mkt Sure, I added all of them. $\endgroup$
    – philsf
    May 11, 2023 at 11:25
  • $\begingroup$ Can you explain time and outcome in a little more detail? Also, the question in your title is perhaps too much of an oversimplification. It can be very useful to include predictors that are known to be related to the response variable in some situations, and less so in others, depending on the causal pathways. $\endgroup$
    – mkt
    May 11, 2023 at 11:48
  • 1
    $\begingroup$ But I agree that if outcome is defined completely by time, it doesn't make sense to use time as a predictor for outcome (or vice versa, for that matter). $\endgroup$
    – mkt
    May 11, 2023 at 11:58
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    $\begingroup$ @mkt Time will be assessed as a secondary outcome (with a linear model), but the main goal is to assess accuracy. Thank you for your response, would you expand on it for an answer? $\endgroup$
    – philsf
    May 11, 2023 at 12:04

1 Answer 1


outcome in this situation is fully determined by time. Another way to say this is that it is simply a re-expression of time on a binary scale. So if you were to model outcome and use time as a predictor, the other predictors would not matter - all the variation in outcome would be fully explained by time.

Actually, that's an oversimplification - it would be worse than this. You would presumably be using a logistic regression, and would encounter the problem of perfect separation.

I would add that outcome does not seem very useful to use as a response variable. You could just model time as a response; taking a continuous value and turn it into a binary throws away useful information. It's very unlikely that the binary outcome variable is a better metric of 'accuracy' than time.


Yes, including group*experience makes sense. As EdM says, it would tell you whether the effect of the treatment (group) on the outcome changes with experience. This is easier to understand if you plot the model output.

Also, if you've measured each participant more than once, you will need to model the non-independence of data points. A mixed model (random intercept and perhaps random slope for participant) would help address this.

  • $\begingroup$ thanks, that's helpful. What about the consideration of group*experience interaction? Does it help explain anything? How would you interpret this, if it turns out to be significant? $\endgroup$
    – philsf
    May 11, 2023 at 12:19
  • $\begingroup$ No, experience is measured in years, as stated. How many years does the participant have performed the procedure for? $\endgroup$
    – philsf
    May 11, 2023 at 12:29
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    $\begingroup$ @philsf not quite. The interaction would examine whether the effect of the experimental manipulation differed as a function of prior experience. The simple additive model would handle "inexperienced participants ... more likely to have a different average time (or accuracy) than experts" on its own, if the effect of the experimental manipulation doesn't depend on prior experience. If you have a large enough data set, it makes sense to include an interaction to check that. Also consider whether your simple linear model of experience is reasonable; a flexible spline fit is often better. $\endgroup$
    – EdM
    May 11, 2023 at 12:43
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    $\begingroup$ @philsf I've updated my answer to address that. I agree with all EdM's comments. $\endgroup$
    – mkt
    May 11, 2023 at 13:21
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    $\begingroup$ @philsf the addition to this answer says what I would have said: there's no downside to including the interaction. For further study, this answer and this answer show plots of situations in which an interaction term between a categorical predictor (group here) and a continuous predictor (experience here) would be important. Harrell's Regression Modeling Strategies is a useful resource on choosing predictors and flexible modeling of continuous predictors. $\endgroup$
    – EdM
    May 11, 2023 at 13:40

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