1. A method of estimating a significance d in a difference between a measurement of a cellular constituent in a first microarray experiment and a measurement of the cellular constituent in a second microarray experiment comprising: computing on a suitably programmed computer d = ( X - Y ) σ X 2 + σ Y 2 + f 2 ⁡ ( X 2 + Y 2 ) wherein X is a brightness of a probe spot representing said cellular constituent in said first microarray experiment; Y is a brightness of a probe spot representing said cellular constituent in said second microarray experiment; σ X 2 is a variance term for X that represents an additive error level in X; σ Y 2 is a variance term for Y that represents an additive error level in Y; f is a fractional multiplicative error level; σ X 2 +f 2 X 2 is an estimated variance for X; and σ Y 2 +f 2 Y 2 is an estimated variance for Y.