Why are there so many glitches in stock market (equity) data?

Why are there so many glitches in stock market data?

The obvious answer is human error, aka GIGO (Garbage In, Garbage Out). Errors are made with stock market data entries.

Web site providers of free data subscribe to services that collect data. There's no real incentive (free?) for the provider to fix errors. The classic example of this is Yahoo Finance though they do not have a monopoly here. You can find a number of "bad data" posts on various Stack BBs and even more if you search the net.

Reliable data comes from two sources:

• Data vendors who check for errors and clean up their database ($ubscription fee)

• The stock exchanges who do the same. While not infallible, Time & Sales is the closest thing to perfect. It will prove the bid and ask every time it changes as well as shares and size for every trade.

You’re mistaking common and frequent. Stock pricing glitches are common, but not frequent. That’s because of the vast volume of data — prices every few seconds for thousands of equities means that there are millions of data points every day. If one out of every ten million of those data points is wrong, then there will be several errors per week, which makes them common. But one error out of every ten million is a very low frequency of errors.