Because all markets are really related in some ways, and you can't judge performance without looking at alternatives. Sometimes, a sector goes down, or up, by i

Because all markets are really related in some ways, and you can't judge performance without looking at alternatives. Sometimes, a sector goes down, or up, by itself, and then we can expect that the reason for the price change has something to do with that sector. The same thing happens when looking at companies: If Home Depot is down 3% but Lowes is also down 3% and the SP500 is down 5% on average, it's a likely explanation that the tumble has nothing to do with HD's outlook. Heck, by some measures, the home improvement retail sector would be doing better than average! The same thing happens when the market is on the way up: Looking at a company or a sector in isolation doesn't let us even speculate on the reasons of the price change. In a site like this, where we care specifically about the health of tech, claiming that the sky is falling for our sector without looking at other sectors would be folly.