![]() We can calculate its power spectrum to determine what frequencies dominate the variance. Plot historical data from New York City posted by Ben Bolker. (2001) show the striking clock-like periodicity of measles in the UK in the pre-vaccine era and how it decays following the introduction of a safe, effective measles vaccine. This independence can be improved – as can the visual quality and interpretability of the plot – by smoothing the periodogram using a kernel smoother (which is generally some sort of weighted running average). The spectral density can be estimated using on object known as a periodogram, which is the squared correlation between our time series and sine/cosine waves at the different frequencies spanned by the series (Venables
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