# What is bias in aerosol data?

I was reading this information related to aerosol, especially aerosol optical depth of the MISR and MODIS instruments. I didn't actually get what bias means in the context of the AOT retrievals of these instruments.

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Bias typically varies from one value to another. For example, p. 65 of the MODIS algorithm documentation (2009) compares MODIS to AERONET sunphotometer values to "validate" the MODIS results. The bias is depicted by the vertical discrepancies between the plotted points and an imaginary diagonal line with equation $y=x$.