The are often inconsistencies between the results of different methods for deriving electron temperatures from line strengths in PNe. Temperatures derived from forbidden lines ratios such as [OIII]4363/5007, are on average higher than those calculated from permitted lines. This is normally explained by assuming the existence of areas of higher temperatures which have more weight in a forbidden line analysis, the so-called ‘temperature fluctuations’. The magnitude of these fluctuations is expressed by the t 2 parameter. See Peimbert (1967). A typical value is about 0.03, corresponding to fluctuations of ∼ 10%. Photoionization models have difficulty in explaining fluctuations of this magnitude (Kingdon & Ferland 1995).