Cohens D Effect Size
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Cohen's d expresses the difference between two group means in standard deviation units, rather than in the original measurement scale. A d of 1.0 means the groups differ by one full standard deviation, making effect sizes comparable across studies using different measurement scales.

Cohen's original convention labels d = 0.2 as a small effect, 0.5 as medium, and 0.8 as large, but these are rough benchmarks, not universal rules; a 'small' effect in medicine can be practically significant, while a 'large' effect in a noisy field might be unremarkable.

No, they measure different things: a p-value tells you whether an observed difference is likely due to chance, while Cohen's d tells you how large that difference is in practical terms. A study can have a statistically significant result with a tiny effect size, especially with a large sample.

Pooling the standard deviations of both groups (weighted by their sample sizes) gives a more stable and representative measure of the variability in the population being studied, rather than relying on one group's variability, which might be atypical.

Yes, the sign simply indicates the direction of the difference, that is, which group had the higher mean. Researchers often report the absolute value when only the magnitude of the effect matters, but the sign is meaningful when direction is part of the hypothesis.