When would we use principal components? What are the two ways to think about PCA? How is a PC represented? Given the first PC, how do you find the second PC? How do we decide how many PCs to use? How many are there?

What is a heat map for correlation? What do the colors of a heat map for data represent? In general, what does a heat map show? What do the dendrograms show on heatmaps? Whats the difference between the dendrograms on the columns and rows of a heatmap?