In cricket, a batsman will be not out if he comes out to bat in an innings and has not been dismissed by the end of the innings. One may similarly describe a batsman as not out while the innings is still in progress.
At least one batsman will be not out at the end of an innings, because once ten batsmen are out, the eleventh will have no partner to bat on with. Two batsmen will be not out if the batting side "declares" in first-class cricket, and often at the end of the scheduled number of overs in limited overs cricket. A batsman further down the batting order than the not-out batsmen will not come out to the crease at all and is noted as did not bat rather than not out; by contrast, a batsman who comes to the crease but faces no-balls is not out. A batsman who retires hurt is considered not out; an uninjured batsman who retires (rare) is considered retired out.
In standard notation a batsman's score is appended with an asterisk to show the not out final status; for example, 10* is read '10 not out'.
Impact on not-out batsmen of the outs (dismissals) component of batting averages
Batting averages are personal and are calculated as runs divided by dismissals, which means that a player who often ends the innings not out may get an inflated batting average, on the face of it. Examples of this include MS Dhoni (78 not outs in ODIs), Michael Bevan (67 not outs in ODIs), James Anderson (74 not outs in 189 Test innings), and Bill Johnston topping the batting averages on the 1953 Australian tour of England.
Two independent counter-factors can mean the simple batting average formula understates performance:
- If not outs were counted as dismissals a usually high-scoring batsman could bat briefly. He may regularly make a low score, not out, facing a low number of balls from a bowler and thus be penalised for factors out of his control.
- A batsman will tend to be at his most vulnerable early in his innings (off the bench) before he has "got his eye in"; as a result, it may be a greater achievement to achieve two scores of 20 not out and 20 (i.e. averaging 60) than to make one score of 40, since in the latter instance the batsman will only have had to deal with one set of variables (see ceteris paribus, all things remaining approximately equal).
These counterbalancing elements have been at the heart of the rationale of keeping the existing simple formula in the 21st century among cricket statisticians, who have used this method of collecting batting averages since the 18th century, after some intervening controversy.