There are many different and wonderful ways to assess the quality of microarray data. Our first analysis is not one of them. Instead, we are going to look at pictures of the chips (well, sort of — they are actually pseudo-images).
Evaluate the following code line-by-line. Some of the code pops up a graphics window, so if you run it all at once you will miss the earlier images. ## Now we want to look at the images of the chips. Basically, we are looking ## for something unusual here; one chip too bright for example. ## Construct image plots for quality control assessment ## Note: that these plots appear dark; they are not log transformed par(oma = c(1,1,3,1)) par(mfrow = c(2,3)) palette.gray <- c(rep(gray(0:10/10), times = seq(1,41, by = 4))) image(soy.ab[,1], transfo = function(x) x, col = palette.gray) image(soy.ab[,2], transfo = function(x) x, col = palette.gray) image(soy.ab[,3], transfo = function(x) x, col = palette.gray) image(soy.ab[,4], transfo = function(x) x, col = palette.gray) image(soy.ab[,5], transfo = function(x) x, col = palette.gray) image(soy.ab[,6], transfo = function(x) x, col = palette.gray) mtext ("> image(soy.ab[,i=1:6], transfo = function(x) x, col = palette.gray)", side = 3, outer = T, cex = .8) ## Construct individual image plots using log intensities ## Note: that these plots appear lighter; they are log transformed ## ## This allows us to see a single microarray chip. par(mfrow = c(1,1)) palette.gray <- c(rep(gray(0:10/10), times = seq(1,41, by = 4))) image(soy.ab[,1], col = palette.gray) title(sub = "> image(soy.ab[,1], col = palette.gray)") ## Repeat above commands with .. soy.ab[,i], i = 2-6 ## Display all six log-transformed image plots in one plot par(oma = c(3,1,3,1)) par(mfrow = c(2,3)) image(soy.ab[,1], col = palette.gray) image(soy.ab[,2], col = palette.gray) image(soy.ab[,3], col = palette.gray) image(soy.ab[,4], col = palette.gray) image(soy.ab[,5], col = palette.gray) image(soy.ab[,6], col = palette.gray) mtext("Image Plots - Hawaii/Resistant vs. Taiwan/Susceptible", side = 3, outer = T) mtext("Top Row: Hawaii/Resistant - Bottom Row: Taiwan/Susceptible", side = 1, outer = T) ## Reset to single screen. par(mfrow = c(1,1))(Complete File)(Rout) You can see the graphics here |
So, what are we looking for here? Essentially, the microarray should be a boring, grayish blob. Any thing at all that peaks your interest is likely to be bad.
Take, for example, the accidental introduction of a coffee ring into an experiment. Or, alternatively, new age designs. Other things to look for would be colour changes from one side to the other, big white holes in the image, and so on, or one array which looks distinctly different from the others.
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