Counting how many subjects have records satisfying a condition
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^longch^ idname [^if^ exp] [^in^ range], ^c(^string^)^

Description
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The command ^longch^ is used with data in which their are several
records per subject. The variable ^idname^ identifies the subjects,
and the string in ^c()^ contains a logical condition. The program
reports the number of subjects for which some records satisfy the
condition; the number for which no records satisfy the condition; and
the number of subjects for which every record satisfies the condition.
In addition the logical variables ^_some^, ^_none^ and ^_every^ which 
flag the records are created. This is useful for dropping all records
belonging to subjects of a particular type, eg those with no records
satisfying the condition.

Options
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^c()^ is not an option - it contains a logical condition

Example
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Consider a longitudinal data set in which each record corresponds to a
visit by a subject and the subject identity is in the variable id. The
command

^longch id, c(height == . )^

results in the output

71  records fulfill the condition height == .

some : 46  subjects have height == . in at least one record
none : 51  subjects have height == . in no records
every: 0  subjects have height == . in every record

In addition three logical variables called ^_some^, ^_none^,
and ^_every^, are created for convenience in further manipulation
(eg dropping or keeping records). These flag all records belonging to
subjects with some records satisfying the condition, and so on.

Authors
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	Michael Hills
        mhills@@regress.demon.co.uk

        Tony Brady
        tbrady@@rpms.ac.uk

Also see
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On-line: help for @count@, @unique@