Setting BRAID Models and Constraints

Basic Selections

There are several ways to specify the space of possible BRAID models that a braidrm fitting function will consider when fitting to a given set of data. Briefly, there are two ways to constrain this space: by fixing parameters to be equal to a constant value to each other (thus reducing the dimensionality of the space of BRAID parameters), and by bounding the range of parameter values being considered, which keeps the same dimensionality but reduces the size of the available space. BRAID fitting functions provide options for both.

The simplest method for controlling the dimensionality of parameters is with the three, named, pre-defined models. Setting the model parameter of a BRAID fitting function to one of "kappa1", "kappa2" (the default), or "kappa3" will select a BRAID model in which potency, Hill slope, and interaction parameters are all allowed to vary freely, as well as the value of the minimal effect E₀. The difference between the three named models is the space of possible maximal effect parameters: "kappa1" specifies a model in which all three maximal effect parameters are constrained to be equal to one another; "kappa2" allows the individual maximal effect parameters E_f, A and E_f, B to vary independently but constrains the overall maximal effect parameter E_f to be equal to the “larger” of the two; and "kappa3" allows all three to vary separately, thus allowing for an overall maximal effect greater than the individual maximal effects. We can see this in action with the synergisticExample dataset:

surface <- synergisticExample

coef(braidrm(measure ~ concA + concB, surface, model="kappa1", getCIs=FALSE))
#>        IDMA        IDMB          na          nb       kappa          E0 
#>  1.03954262  1.04805325  2.94534132  2.46109206  2.16620128 -0.03002773 
#>         EfA         EfB          Ef 
#>  1.00570420  1.00570420  1.00570420
coef(braidrm(measure ~ concA + concB, surface, model="kappa2", getCIs=FALSE))
#>        IDMA        IDMB          na          nb       kappa          E0 
#>  1.03979196  1.02589884  2.91156457  2.49903773  2.12580099 -0.03002222 
#>         EfA         EfB          Ef 
#>  1.00804226  0.98482987  1.00804226
coef(braidrm(measure ~ concA + concB, surface, model="kappa3", getCIs=FALSE))
#>       IDMA       IDMB         na         nb      kappa         E0        EfA 
#>  1.0398157  1.0259278  2.9115964  2.4991254  2.1258819 -0.0300150  1.0080412 
#>        EfB         Ef 
#>  0.9848315  1.0080412

Note that while the third model allows the maximal effect parameter E_f to differ, in this case (and in many cases) the best fitting surface is still one in which E_f is equal to one of the individual maximal effects.

Precise Model Selection

In some cases, the user may wish to consider models beyond the basic three. The model parameter can therefore pick out explicitly which parameters should be varied freely while fitting. If model is a numeric vector containing a subset of the indices 1 through 9, all indices that are included will be allowed to vary during fitting, and those that are not included will be fixed. For example:

startingParameter <- c(2,2,2,2,0,0,1,1,1)

bfit <- braidrm(measure ~ concA + concB, surface,
                model=c(1,3,5,6,7,8), start=startingParameter,
                getCIs=FALSE)
coef(bfit)
#>       IDMA       IDMB         na         nb      kappa         E0        EfA 
#> 1.15495135 2.00000000 2.89447474 2.00000000 3.68261342 0.03724723 1.00290174 
#>        EfB         Ef 
#> 1.02997920 1.02997920

bfit <- braidrm(measure ~ concA + concB, surface,
                model=c(1,2,5,6,9), start=startingParameter,
                getCIs=FALSE)
coef(bfit)
#>        IDMA        IDMB          na          nb       kappa          E0 
#>  0.99413424  1.03117464  2.00000000  2.00000000  2.25103927 -0.07513876 
#>         EfA         EfB          Ef 
#>  1.02845985  1.02845985  1.02845985

In the first example, parameters 2 and 4 (ID_M, B and n_b) are absent from the model vector, so they are fixed at the value of the starting parameter. (When leaving any of the first six parameters fixed, it is always best to provide an explicit starting vector to ensure that the fixed values are sensible. BRAID fitting functions will try to automatically generate a starting vector, but such values are not guaranteed to be near what the user wants.) In the second example, it is parameters 3 and 4 (the two Hill slopes) which have been omitted, so they are held constant at the given starting value. Note however, that while parameter 9 (E_f) is absent from the first example, and parameters 7 and 8 (E_f, A and E_f, B) are absent from the second, they are not set equal to the values of the starting parameter. That is because the maximal effect parameters are handled differently from the other six.

Maximal Effect Links

While the first six BRAID parameters can either vary freely or be fixed at a constant, the maximal effect parameters E_f, A, E_f, B and E_f behave a little differently. This is because there are many cases where it is less useful to fix them at a constant value than it is to fix them equal to each other. For example, while it is possible that one might want to fit a BRAID surface where the individual maximal effect parameters E_f, A and E_f, B are fixed a constant 50% efficacy but the overall maximal effect E_f could be anything (and BRAID fitting functions can support that), it is much more likely that one would want to fit a surface where all three maximal effect parameters are held equal to one another. Both circumstances involve E_f varying freely, and the individual maximal effects being constrained, so how do we distinguish between the two? This is done with the links parameter.

links picks out several scenarios in which BRAID parameters are not varying freely, but are held equal to each other in one way or another. It is a character string with one of the following values:

"AB": Both individual maximal effect parameters E_f, A and E_f, B are held equal to the maximal effect parameter E_f. Thus index 9 must be present in the model vector, and indices 7 and 8 must be absent.
"A": E_f, A must be equal to the overall maximal effect E_f. Index 9 must be present in the model vector and index 7 must be absent; if index 8 (E_f, B) is absent, it will be held at a constant starting value.
"B": E_f, B must be equal to the overall maximal effect E_f. Index 9 must be present in the model vector and index 8 must be absent; if index 7 (E_f, A) is absent, it will be held at a constant starting value.
"F": The overall maximal effect parameter E_f is constrained to be equal to the larger of the two individual maximal effect parameters. Index 9 must be absent from the model vector, and one or both of 7 and 8 must be present.
"" (the empty sting): There are no links between parameters. Any parameter indices which are absent result in the corresponding parameter being held constant at the starting value.

Note that none of these value is marked as a default. That is because, when left unspecified, links is chosen by the BRAID fitting function based on the parameters included in model. Models containing all or none of the maximal effect parameters assume a links value of ""; models containing one or both of 7 and 8 (E_f, A and E_f, B) but not 9 (E_f) assume a links value of "F"; and models containing parameter 9 but neither of 7 or 8 assume a links value of "AB". It has been our experience that these defaults are desired in the vast majority of cases.

Parameter Bounds

Even in cases where a parameter should be fit and determined from the data, it is often desirable to place certain boundaries on the value of that parameter. The user may wish to ensure that predicted drug effects never drop below 0, or that the dose of median effect of a drug is held to a known range of values. This is accomplished using the lower and upper parameters. If included, these numeric parameters contain a vector of values specifying the lowest or highest a given parameter may go. They can be full length-9, or the same length as the relevant model vector, in which case they will be mapped to the correct freely varying parameter in the model. For example:

bfit <- braidrm(measure ~ concA + concB, surface,
                model=c(1:5,6,7,8), start=startingParameter,
                upper = c(0.75,0.75,NA,NA,NA,NA,0.7,NA,NA),
                getCIs=FALSE)
coef(bfit)
#>        IDMA        IDMB          na          nb       kappa          E0 
#>  0.75000000  0.75000000  4.31656699  2.06846665  1.28667298 -0.07142398 
#>         EfA         EfB          Ef 
#>  0.70000000  1.00565462  1.00565462

bfit <- braidrm(measure ~ concA + concB, surface,
                model=c(1:5,6,9), start=startingParameter,
                lower = c(NA,NA,3,3,NA,0,0),
                getCIs=FALSE)
coef(bfit)
#>      IDMA      IDMB        na        nb     kappa        E0       EfA       EfB 
#> 1.0495307 1.0756685 3.0153978 3.0000000 2.1204270 0.0000000 0.9994638 0.9994638 
#>        Ef 
#> 0.9994638

In the first example, upper is a length 9 vector, specifying that parameters 1 and 2 (ID_M, A and ID_M, B) cannot go higher than 0.75, and that parameter 7 (E_f, A) cannot go higher than 0.7. In the second, lower is the same length as model, so its values correspond to just those indices. The third and fourth model parameters (which are 3 and 4, n_a and n_b) can go no lower than 3; whereas the sixth and seventh model parameters (6 and 9, so E₀ and E_f) can go no lower than 0. And indeed the best fit vectors in both cases abide by these constraints.

Direction Constraint

The parameters that tend to be bounded most often are the minimal and maximal effect parameters, as these are often real-world measurable quantities with natural constraints on them. BRAID fitting functions also offer one additional boundary to place on these parameters. The direction parameter can specify that BRAID fitting functions should only consider models that change in one particular direction. Setting the parameter to 1 indicates that considered models should always be increasing, so the maximal effect parameters must be numerically greater than or equal to than the minimal effect parameter. Setting direction to −1 indicates the opposite, that the maximal effect parameters considered must always be numerically less than or equal to the minimal effect parameter. Setting direction equal to 0 (the default) will consider surfaces changing in either direction. This can be particularly important when fitting a large number of related surfaces that you wish to compare, as having surfaces that change effect in opposite directions can make analysis much more complex.