Offer your thoughts or opinion on the input financial query or topic using your financial background.

Why is it important to research a stock before buying it?
To a certain degree "the only sure thing I know is the price I paid for the stock is the fair price at the time I buy it" is absolutely right, by definition, and by the law of the free and efficient market and forces of supply and demand, freedom of public information about share price sensitive information, etc, etc, etc, and you've made a good point that eludes many investors I'd say. However, in practise, the market has many participants, and they will all be arriving at a different idea of what the "fair price" is by way of a slightly different analysis and slightly different information. In theory they all have the same information, but unfortunately in practise there is always some disparity. When one participant feels a stock is undervalued though the last thing they want to do is say so, instead they will start buying stock. They might feel it is undervalued by 20%, but that doesn't mean they'll keep buying and buying until it gets to 20%, they might push the price up just a little, then let the price drift down again, buy some more, relax, buy some more, etc. Over time the price will rise of course because the supply will become weaker, but even if the participant is correct about the 20% the price might have only risen 7% by the time they acquire all the stock they want given their risk models, market exposure and margin guidelines, etc, and it might be more than a year later before the price has actually risen to 20%, presumably because more and more other market participants have come to the same conclusion. The opposite can obviously also happen, a participant might dump stock it feels is over valued long before it hits the values it believes in. So right away you can see that pricing might not really reflect value, or "fair price".