Cohen’s d is a statistic that helps to understand the size of an effect, like the difference in age, by standardizing the difference between two group means. It’s calculated by taking the difference between the two means (for example, the average age of Black and White individuals killed by police) and dividing it by the combined standard deviation of both groups.
This standard deviation is a “pooled” value, which means it takes into account the number of people in each group and their respective standard deviations to get an overall measure of variability.
In the case we’re looking at, Cohen’s d was calculated to be 0.577485. This value is considered to show a medium effect size based on standard guidelines. What this means is that the age difference of 7.3 years between Black and White individuals killed by police is neither large nor small, but moderate. It’s statistically significant enough to be noticeable and not due to chance, but it’s not an overwhelmingly large difference.