Well most of the time if there are more shares rising than falling, the index goes up.
And where there is more shares falling than rising, the index goes down.
But sometimes, there are more shares rising than falling and the index still goes down. This is unusual but will happen when a couple of stocks with large index weightings fall although the rest of the market is mostly healthy.
And conversely when a few large cap stocks rise strongly, the index may be up although in fact a majority of the stocks in the market may be down.
Its at these times that people who rely on the major index for their indication of market strength may be somewhat misled.
