This function produces a reduced matrix of capture histories from a complete one by merging together some capture occasions.

It can also be used to change the format of a capture-recapture data set with complete capture histories: it transforms a data set with one row per captured unit to a data set with one row per capture history followed by its frequency.

`periodhist(X, dfreq=FALSE, vt, drop=TRUE)`

X

The matrix of the observed capture histories (see `Rcapture-package`

for a description of the accepted formats).

dfreq

A logical. By default FALSE, which means that `X`

has one row per unit. If TRUE, it
indicates that the matrix `X`

contains frequencies in its last column.

vt

A vector containing the numbers of capture occasions for each pooled capture occasions. The length of this vector equals the number of capture occasions in the reduced matrix (noted \(I\)).

drop

A logical, by default TRUE, meaning that in the output matrix the unobserved
capture histories (frequency of 0) are omitted. To keep them,
`drop`

must be set to FALSE.

A \(2^I-1\) by \(I+1\) matrix of all the pooled capture histories,
with their observed frequencies. If `drop=TRUE`

(the default), the number of rows
of this matrix is in fact \(2^I-1\) minus the number of unobserved capture histories.

This function is useful when using an open population model to analyze a robust design data set. It can be used to reduce the data set to one observation per primary period. A one represents a unit caught at least once during the period and a zero a unit never caught.

It is also useful for experiments with a large number of capture occasions but a limited number of catches,
especially when there is no capture on some occasions. In such cases, one can pool together some capture occasions
with the `periodhist`

function.

If a data matrix produced by the `periodhist`

function is given as an argument to a Rcapture function,
the `dfreq`

argument must be set to `TRUE`

.

Baillargeon, S. and Rivest, L.P. (2007) Rcapture: Loglinear models for capture-recapture in R. *Journal of Statistical Software*, **19**(5), http://www.jstatsoft.org/v19/i05.

```
# NOT RUN {
# mvole data set aggregated per primary period
mvole.op <- periodhist(mvole, vt = rep(5, 6))
openp(mvole.op, dfreq = TRUE)
# }
```

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