Yesterday's market activity was unusual, but highly instructive. It was a great illustration of the different composition and calculation methods of the big three indices -- Dow's up about .6%, S&P down a bit over 1%, and NASDAQ down about 3%.
So how'd that happen? It's all in the definitions.
(Note: The nitty gritty calculations change every day. The numbers quoted below are the latest ones I could find on short notice. While they will not be pluperfectly accurate at the close of business today, they're still valid for purposes of this discussion.)
Dow Jones 30 Industrials
The Dow is 30 companies. This is important: It's weighted by share price, not market cap. It's also diversified across a lot of industries.
A bit of foreshadowing here -- tech is a component, but not a massively outsized one.
Still, even at the Dow (remember, it's calculated based on share price, not market cap), tech has a bit of outsized influence because tech companies tend to have low to mid 3-figure share prices.
S&P 500
Obviously, 500 companies. But their individual influence on the index is determined by their market capitalization, not their share price. Market cap = (share price) x (# of shares outstanding).
This means huge companies have far more influence than smaller ones. How much more?
5 Largest S&P Companies By Market Cap = 26% of the index. IOW, the other 495 companies are 74%
10 Largest S&P Companies by Market Cap = 36% of the index. IOW, the other 490 companies are 64%
Tech accounts for 26% of the index.
NASDAQ 100
Biggest 100 Companies listed on the NASDAQ exchange. Also weighted by Market Cap.
5 Largest NASDAQ Companies By Market Cap = 36% of the index. The other 95 companies are 64%
10 Largest NASDAQ Companies by Market Cap = 52% of the index. The other 90 companies are 48%
Tech accounts for 58% of the index. NVIDIA and Microsoft alone account for about 17%
Point of all that geekiness? Tech drives the NASDAQ. It has a big influence on the S&P, but not so much as it has on the NASDAQ. It's one of a bunch of components of the Dow, and has less influence there.
So NVIDIA was the story yesterday. Lost 17% of its value, or about $600 Billion dollars. It alone accounted for a huge chunk of the overall decline in the S&P and NASDAQ.
And BamaNation was spot on in that this move isn't a reason to change your investment allocation. It's very seldom that market events on any one day are.
What is a good reason to change your allocation? A change in your risk tolerance. Usually, that involves a change in your time horizon. It could something happy....like you just turned 55 and are 10 years out from retirement. Or just life...like a child turning 10, and you have about one business cycle before you have to pay for college. It could be something sad....like a bad diagnosis for your or your spouse.
Regardless, the point is that it's not often a good idea to change your allocation in response to moves in the market. That's also known as chasing the market, really means you're buying high and selling low, and almost never works out. A good reason to change is because something in your own personal life has changed. Or it's part of a periodic rebalancing of your portfolio.
So unless something changed for you personally, "Don't just do something....stand there!"