# All statistics and graphs for U Chart Diagnostic

Find definitions and interpretation guidance for every statistic and graph that is provided with the U chart diagnostic test.

## Poisson probability plot

The probability plot for the U chart diagnostic test shows how well your data fit a Poisson distribution.

### Interpretation

If your data follow a Poisson distribution closely, then the plotted points fall close to the diagonal reference line.

## Ratio of observed variation to expected variation

The ratio of observed variation to expected variation compares the variation in your data to the variation that you would expect based on a Poisson distribution. The ratio is expressed as a percentage.

### Interpretation

A ratio that is close to 100% indicates that your data exhibit the expected amount of variation for a Poisson distribution. Compare the ratio to the 95% upper confidence limit or the 95% lower confidence limit to determine whether your data exhibit significant overdispersion or significant underdispersion:
• If the ratio is greater than the upper confidence limit, then your data exhibit significant overdispersion.
• If the ratio is less than the lower confidence limit, then your data exhibit significant underdispersion.