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Should my effect size be positive or negative when the control group gains more weight than the experimental group (when it is hypothesised that the experimental group would lose more weight i.e., the difference between groups is due to more weight gain in the control rather than weight loss int the experimental group)?

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As long as your wording matches the sign of the effect size, it doesn't matter.

"Women weigh 15 pounds less than men" is exactly the same as "Men weigh 15 pounds more than women"

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thanks! The data point will enter a meta-analysis so there will not be any wording! I just wanted to check that it is ok to claim that the interventon group did better than the control group (but this was becuase the control group gained weight rather than the intervention group losing weight). I think from what you say it is fine to assert that the intervention group did better. thank you. – user42327 Mar 21 '14 at 15:16

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