seq2binomial {VGAM} | R Documentation

## The Two-stage Sequential Binomial Distribution Family Function

### Description

Estimation of the probabilities of a two-stage binomial distribution.

### Usage

    seq2binomial(lprob1 = "logitlink", lprob2 = "logitlink",
                 iprob1 = NULL,    iprob2 = NULL,
                 parallel = FALSE, zero = NULL)

### Arguments

`lprob1, lprob2` | Parameter link functions applied to the two probabilities, called p and q below. See `Links` for more choices.
`iprob1, iprob2` | Optional initial value for the first and second probabilities respectively. A `NULL` means a value is obtained in the `initialize` slot.
`parallel, zero` | Details at `Links`. If `parallel = TRUE` then the constraint also applies to the intercept.

### Details

This VGAM family function fits the model described by Crowder and Sweeting (1989) which is described as follows. Each of m spores has a probability p of germinating. Of the y1 spores that germinate, each has a probability q of bending in a particular direction. Let y2 be the number that bend in the specified direction. The probability model for this data is P(y1,y2) =

{choose(m,y1)} p^{y1} (1-p)^{m-y1} {choose(y1,y2)} q^{y2} (1-q)^{y1-y2}

for 0 < p < 1, 0 < q < 1, y1=1,…,m and y2=1,…,y1. Here, p is `prob1`, q is `prob2`.

Although the Authors refer to this as the bivariate binomial model, I have named it the (two-stage) sequential binomial model. Fisher scoring is used.

### Value

An object of class `"vglmff"` (see `vglmff-class`). The object is used by modelling functions such as `vglm` and `vgam`.

### Note

The response must be a two-column matrix of sample proportions corresponding to y1 and y2. The m values should be inputted with the `weights` argument of `vglm` and `vgam`. The fitted value is a two-column matrix of estimated probabilities p and q. A common form of error is when there are no trials for y1, e.g., if `mvector` below has some values which are zero.

### Author(s)

Thomas W. Yee

### References

Crowder, M. and Sweeting, T. (1989). Bayesian inference for a bivariate binomial distribution. Biometrika, 76, 599–603.

### See Also

`binomialff`, `cfibrosis`.

### Examples

    sdata <- data.frame(mvector = round(rnorm(nn <- 100, m = 10, sd = 2)),
                        x2 = runif(nn))
    sdata <- transform(sdata, prob1 = logitlink(+2 - x2, inverse = TRUE),
                              prob2 = logitlink(-2 + x2, inverse = TRUE))
    sdata <- transform(sdata, successes1 = rbinom(nn, size = mvector,    prob = prob1))
    sdata <- transform(sdata, successes2 = rbinom(nn, size = successes1, prob = prob2))
    sdata <- transform(sdata, y1 = successes1 / mvector)
    sdata <- transform(sdata, y2 = successes2 / successes1)
    fit <- vglm(cbind(y1, y2) ~ x2, seq2binomial, weight = mvector,
                data = sdata, trace = TRUE)
    coef(fit)
    coef(fit, matrix = TRUE)
    head(fitted(fit))
    head(depvar(fit))
    head(weights(fit, type = "prior"))  # Same as with(sdata, mvector)
    # Number of first successes:
    head(depvar(fit)[, 1] * c(weights(fit, type = "prior")))
    # Number of second successes:
    head(depvar(fit)[, 2] * c(weights(fit, type = "prior")) *
                              depvar(fit)[, 1])

[Package VGAM version 1.1-1 Index]