Application of the Potthoff-Whittinghill technique to detecting temporal clustering of influenza hospitalisations among children aged 0–4 years in California^a during the flu seasons from 2006–07 to 2011–12 inclusive.

Notes: Based on locations participating in the California Emerging Infections Program. aPW is the one-step estimate of β, the extra-Poisson variation, calculated as S/i(0) in the notation of Muirhead [22]. bSE is the standard error of PW in the absence of extra-Poisson variation, calculated as 1/√i(0) in the notation of Muirhead [22]. cp-values have been calculated using 10000 simulations of PW, assuming Poisson variation. All p-values are one-sided.