What is the difference between a null and a zero value?
A null value is a missing or excluded observation, recorded as a gap in a worksheet or as the asterisk * symbol. The internal code for a missing value is 3E+300, which would also be treated as missing/null if you entered it as an observed value. Zero must always be entered as 0 or 0.0 or 0.0e0 in order for it to be treated as an observation of zero – but remember that in categorical analysis, such as counts in a contingency table, some researchers may refer to zero as null response.
