Conceptual definitions important in understanding

the language of climatic change

Mean – the average value

Normal – defined as the mean for the 30 yr period ending in the last year of the last decade (currently 1981-2000)

Standard deviation – a statistical measure, obtained mathematically, of variations from the mean. Works well for precipitation and expresses the range of annnual precipitation values that can be expected 67% of the time, based upon past history. SF: 22.68” +/- 6”

Coefficient of varaition – standard deviation expressed as a faction or percentage. SF: 6/23 X 100 = 26%

Return period -- a measure fo the expected frequency of occurrence of an event based upon analysis of past records. For example, a return period of 100 yr indicates that, based upon an analysis of past records, the given event has a 1% chance of occurring in any given year (also called “the 100 yr event”).

Correlation – the occurrence of one event is linked, by statistical test, to the occurrence of another event, for example. Correlations range from +1.0 (every “time” one event occurs, the other event does too) to –1.0 (every “time” one event occurs, the other event does not occur and vice versa). No cause and effect is implied by correlation.

Statistical significance – in the context of correlation, the given relationship between the events is not due to chance alone and there is a systematic reason for the relationship. Determined by statistical test.