Interpretation of the pattern in rate ratios across strata

Area of Science:

• Epidemiology
• Biostatistics

Background:

• The ratio of disease rates across strata is a common summary statistic in epidemiology.
• Interpreting these ratios can be challenging as their patterns may not directly reflect disease rate changes.

Purpose of the Study:

• To clarify the relationship between disease rate ratios and actual disease rate patterns.
• To explain why ratio patterns can sometimes appear to contradict disease rate trends.

Main Methods:

• Utilized algebraic manipulation to analyze the mathematical relationship between rate ratios and rate differences.
• Examined hypothetical scenarios with parallel disease rate declines in different populations.

Main Results:

• Demonstrated that the pattern of the rate ratio is mathematically dependent on the rate difference relative to the endemic disease rate.
• Showed that when disease rates decline in parallel, the rate ratio increases over strata, despite a constant rate difference.

Conclusions:

• Rate ratio patterns do not solely describe changes in disease rates.
• Ratio patterns are influenced by the magnitude of exposure effect relative to the baseline disease rate in the unexposed population.