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Fuel Moisture Stress Index (FMSI) was first proposed by Yool (2001), as a way of measuring the variation in live fuel moisture and thus its relative rigor or stress.

The original per pixel FMSI, derived using the formula for the standard score, is computed using the pixel's NDVI recorded for a given time step/period in a given year, and the NDVI mean and NDVI standard deviation for the same time step/period across the entire time series. The FMSI of each pixel is thusly placed in its distinctive statistical location in a 'normal distribution' that is defined by all a given pixel's FMSI values for the same time period (i.e., same 'Julian' period) across the entire time series.

Tracking the FMSI for each year for a given pixel's 'phenoperiod' roughly yields, with some statistical rigor, interannual trends in (as we like to say) 'climate forcing' and are amenable to inferential analyses.

Burgan and Hartford (1993) computed Relative Greenness similarly in terms of using the same 'phenoperiod', producing descriptive statistics for NDVI in the context of live fuel moistures. The FMSI took the next step, converting the NDVI values to standard scores to enable inferential statistics and timeseries analyses.