Comparison of parameters

Compare parameters from different assays, either by means of ratios or differences.

compParm(object, strVal, operator = "/", vcov. = vcov, od = FALSE,
  pool = TRUE, display = TRUE)

Arguments

object	an object of class 'drc'.
strVal	a name of parameter to compare.
operator	a character. If equal to "/" (default) parameter ratios are compared. If equal to "-" parameter differences are compared.
vcov.	function providing the variance-covariance matrix. vcov is the default, but sandwich is also an option (for obtaining robust standard errors).
od	logical. If TRUE adjustment for over-dispersion is used.
pool	logical. If TRUE curves are pooled. Otherwise they are not. This argument only works for models with independently fitted curves as specified in drm.
display	logical. If TRUE results are displayed. Otherwise they are not (useful in simulations).

Value

A matrix with columns containing the estimates, estimated standard errors, values of t-statistics and p-values for the null hypothesis that the ratio equals 1 or that the difference equals 0 (depending on the operator argument).

Details

The function compares actual parameter estimates, and therefore the results depend on the parameterisation used. Probably it is most useful in combination with the argument collapse in drm for specifying parameter constraints in models, either through data frames or lists with formulas without intercept (-1).

Examples

# Fitting a model with names assigned to the parameters!
spinach.m1 <- drm(SLOPE~DOSE, CURVE, data = spinach,
fct = LL.4(names = c("b", "lower", "upper", "ed50")))

## Calculating ratios of parameter estimates for the parameter named "ed50"
compParm(spinach.m1, "ed50")
#> 
#> Comparison of parameter 'ed50' 
#> 
#>     Estimate Std. Error t-value  p-value   
#> 1/2  1.89836    0.71185  1.2620 0.210398   
#> 1/3  1.30730    0.55416  0.5545 0.580668   
#> 1/4  9.09638    2.46866  3.2797 0.001508 **
#> 1/5  8.51523    2.33645  3.2165 0.001836 **
#> 2/3  0.68865    0.29081 -1.0706 0.287360   
#> 2/4  4.79171    1.28835  2.9431 0.004189 **
#> 2/5  4.48557    1.21960  2.8580 0.005361 **
#> 3/4  6.95813    2.32206  2.5659 0.012045 * 
#> 3/5  6.51359    2.18960  2.5181 0.013674 * 
#> 4/5  0.93611    0.07814 -0.8176 0.415868   
#> ---
#> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

## Calculating differences between parameter estimates for the parameter named "ed50"
compParm(spinach.m1, "ed50", "-")
#> 
#> Comparison of parameter 'ed50' 
#> 
#>      Estimate Std. Error t-value  p-value   
#> 1-2  0.849425   0.539400  1.5748 0.119027   
#> 1-3  0.421932   0.658506  0.6407 0.523414   
#> 1-4  1.597629   0.478341  3.3399 0.001246 **
#> 1-5  1.584161   0.478432  3.3112 0.001365 **
#> 2-3 -0.427493   0.516885 -0.8271 0.410521   
#> 2-4  0.748204   0.249701  2.9964 0.003580 **
#> 2-5  0.734736   0.249876  2.9404 0.004221 **
#> 3-4  1.175696   0.452799  2.5965 0.011095 * 
#> 3-5  1.162229   0.452896  2.5662 0.012034 * 
#> 4-5 -0.013467   0.017174 -0.7842 0.435128   
#> ---
#> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1