# Wald Test for Stratification Factors

### Description

It performs a Wald test to test the hypothesis of equal spline coefficients among strata in the approximation of baseline risk function.

### Usage

1 | ```
wald.strata(fit)
``` |

### Arguments

`fit` |
an object of class |

### Value

an object of class `wald.strata`

with components:

`alternative` |
a character string naming the alternative. |

`Result` |
a numeric matrix with the results of the Wald test. |

### Note

This test is valid when the same knots have been used across strata.

### Author(s)

Dimitris Rizopoulos d.rizopoulos@erasmusmc.nl

### References

Rizopoulos, D. (2012) *Joint Models for Longitudinal and Time-to-Event Data: with
Applications in R*. Boca Raton: Chapman and Hall/CRC.

### Examples

1 2 3 4 5 6 7 8 9 | ```
## Not run:
fitLME <- lme(log(serBilir) ~ drug * year - drug, random = ~ year | id,
data = pbc2)
fitSURV <- coxph(Surv(years, status2) ~ drug + strata(hepatomegaly),
data = pbc2.id, x = TRUE)
fit.pbc <- jointModel(fitLME, fitSURV, timeVar = "year", method = "spline-PH-aGH")
wald.strata(fit.pbc)
## End(Not run)
``` |