It is very easy to calculate.
PE ratio = share price / earnings per share (EPS)
In simple terms, shares with high PE ratios are seen as being expensive whilst those with low ones are seen as being cheaper.
Despite its simplicity, PE ratio has many pitfalls that can give investors a misleading view of how cheap or expensive some share really are.
The PE ratio's drawbacks are all to do with the "E" or EPS, part of the calculation
1. EPS is easy to manipulate.
Companies can boost EPS by changing accounting policies.
For example, they can extend the useful lives of fixed assets such as plant and machinery, which lowers the depreciation expense and boosts profits.
2. EPS says nothing about the quality of profits.
It doesn't take into account whether profits have changed due to sales of existing products or services - the best source of profits growth - or whether the company has invested heavily in new assets or bought another company (acquisition).
Share buybacks boost EPS by shrinking the number of shares outstanding, even if profits are static or shrinking. Buyback can be done when the shares are expensive. By paying too much, a large chunk of shareholder value is destroyed; the cash spent is wasted.
3. EPS may not resemble true cash profits.
Quite often a company's true cash profits are significantly more or less than its EPS (more often less).
4. EPS may be based on profits that are unsustainably high or temporarily low.
This means that the PE ratio could be misleadingly low or high.
This is a particular problem for cyclical companies.
For the above reasons, EPS can be unreliable and you should not rely on PE alone.
Once again, PE has may pitfalls that can give investors a misleading view of how cheap or expensive some shares really are.