At least one batter is not out at the end of every innings, because once ten batters are out, the eleventh has no partner to bat on with so the innings ends. Usually two batters finish 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.
Batters further down the batting order than the not out batters do not come out to the crease at all and are noted as did not bat rather than not out; by contrast, a batter who comes to the crease but faces no balls is not out. A batter who retires hurt is considered not out; an uninjured batter who retires (rare) is considered retired out.
In standard notation a batter's score is appended with an asterisk to show the not out final status; for example, 10* means '10 not out'.
Impact on batting averages
Batting averages are personal and are calculated as runs divided by dismissals, so 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 (82 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 batter 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 batter will tend to be at his most vulnerable early in his innings 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 40) than to make one score of 40, since in the latter instance the batter 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.