For a command-of-evidence question involving an interaction, identify the untreated baseline and calculate each treatment's separate effect. Keep reductions and resulting means distinct, compute the combined result predicted by the model described in the passage, and compare that prediction with the observed combined-treatment result.
Hints
- Hint 1
Find the baseline
Use the plots receiving neither warming nor mowing as the point of comparison.
- Hint 2
Calculate separate effects
Determine how much warming alone and mowing alone each changed the mean number of visits.
- Hint 3
Keep quantities distinct
Add the two separate declines, convert that predicted decline into a predicted mean, and then compare the prediction with the observed combined-treatment mean.
Step-by-step
- Step 1
Identify the untreated baseline
The ambient, unmowed plots received a mean of 22 visits. This value is the baseline for calculating each treatment's effect.
- Step 2
Calculate the separate reductions
Mowing alone reduced visits from 22 to 17, a reduction of 5. Warming alone reduced visits from 22 to 18, a reduction of 4. If these effects were additive, their combined reduction would be 9, leaving an expected mean of 13 visits.
- Step 3
Compare the prediction with the result
The warmed, mowed plots received a mean of 9 visits, a reduction of 13 from the baseline. Because 13 is 4 greater than the additive reduction of 9, the treatments amplified each other's effects.