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The Dangers of Drawing Cohort Profiles From Period Data: A Research Note
Alyson A van Raalte1,2, Ugofilippo Basellini1, Carlo Giovanni Camarda3
1Max Planck Institute for Demographic Research, Rostock, Germany.
Demography
|November 15, 2023
Summary
Demographic measures fluctuate artificially due to timing and cohort-size bias when calculated from age-period data diagonals. These biases can lead to implausible mortality forecasts, even with detailed data.
Area of Science:
- Demography
- Population Studies
- Statistical Modeling
Background:
- Cohort profiles and forecasts are commonly derived from age-period data grids in demography.
- Existing methods may introduce artificial fluctuations in demographic measures.
Purpose of the Study:
- To demonstrate artificial fluctuations in demographic measures derived from age-period data diagonals.
- To quantify the magnitude of timing and cohort-size biases.
- To illustrate the potential for implausible mortality prediction intervals from Lee-Carter models.
Main Methods:
- Analysis of demographic measures calculated from the diagonals of age-period rate grids.
- Estimation of bias magnitudes.
- Application of Lee-Carter methods to age-period data for mortality forecasting.
Main Results:
- Demographic measures exhibit artificial fluctuations due to timing and cohort-size bias.
- These biases are substantial, impacting measures even when using single-age, single-year data.
- Prediction intervals for cohort mortality indicators can become implausible.
Conclusions:
- Data manipulation in age-period grids can induce biases in cohort measures.
- Overinterpretation of trends influenced by these biases is a significant risk.
- Careful consideration of data derivation methods is crucial for accurate demographic analysis and forecasting.
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