P/E ratios

I was just comparing P/E ratios for the same company(ies) in various publications.

	Moneycontrol	Rediff Money	Times-of-India
Bharti Airtel	30.02	42.67	29.7
Reliance Industries	20.49	32.48	19.8

So, there is a clear difference. Moneycontrol and Times calculate PE based on trailing twelve
months whereas Rediff calculates based on exactly an year ago.

For e.g. a company A has following quarterly earnings :
Mar'06 : x
June'06 : y
Sep'06 : z
Dec'06 : w
Mar'07 : p
June'07 : q
Sep'07 : r
Dec'07 : s

So, to calculate P/E today(Feb'2008), Rediff would take E = x+y+z+w but Moneycontrol
would take it as E = p+q+r+s.