In `simstudy`

, there are at least two ways to define a
binary data generating process. The first is to operate on the scale of
the proportion or probability using the *identity* link. This
allows users to define a data generating process that reflects
assumptions about risk ratios and risk differences when comparing two
groups defined by an exposure or treatment. However, this process can
become challenging when introducing other covariates, because it can be
difficult to constrain the probabilities so that they fall between 0 and
1.

The second approach works on the log-odds scale using a
*logit* link, and is much more amenable to accommodating
covariates. Unfortunately, this comes at the price of being able to
easily generate specific risk ratios and risk differences, because all
parameters are log-odds ratios. The overall (marginal) prevalence of an
outcome in a population will vary depending on the distribution of
covariates in that population, and the strengths (both absolute and
relative) of the association of those covariates with the outcome. That
is, the coefficients of a logistic model (including the intercept)
determine the prevalence. The same is true regarding the risk ratio and
risk difference (if there is one particular exposure or treatment of
interest) and the AUC.

Here we start with the simplest case where we have a target marginal
proportion or prevalence, and then illustrate data generation with three
other target statistics: **risk ratios**, **risk
differences**, and **AUCs**.

In this first example, we start with one set of assumptions for four
covariates \(x_1, x2 \sim N(0, 1)\),
\(b_1 \sim Bin(0.3)\), and \(b_2 \sim Bin(0.7)\), and generate the
outcome *y* with the following data generating process:

\[ \text{logit}(y) = 0.15x_1 + 0.25x_2 +
0.10b_1 + 0.30b_2\]

```
library(simstudy)
library(ggplot2)
library(data.table)
coefs1 <- c(0.15, 0.25, 0.10, 0.30)
d1 <- defData(varname = "x1", formula = 0, variance = 1)
d1 <- defData(d1, varname = "x2", formula = 0, variance = 1)
d1 <- defData(d1, varname = "b1", formula = 0.3, dist = "binary")
d1 <- defData(d1, varname = "b2", formula = 0.7, dist = "binary")
d1a <- defData(d1, varname = "y",
formula = "t(..coefs1) %*% c(x1, x2, b1, b2)",
dist = "binary", link = "logit")
set.seed(48392)
dd <- genData(500000, d1a)
dd
```

```
## id x1 x2 b1 b2 y
## 1: 1 0.29 0.390 0 1 1
## 2: 2 0.76 -0.925 0 0 0
## 3: 3 -1.47 0.939 0 0 1
## 4: 4 1.92 0.560 0 1 1
## 5: 5 1.40 -0.238 0 1 0
## ---
## 499996: 499996 -0.32 0.367 0 0 0
## 499997: 499997 -1.08 2.152 0 0 0
## 499998: 499998 -1.10 0.380 1 0 0
## 499999: 499999 0.56 -1.042 0 1 0
## 500000: 500000 0.52 0.076 0 1 1
```

The overall proportion of \(y=1\) in this case is

`## [1] 0.56`

If we have a desired marginal proportion of 0.40, then we can add an intercept of -0.66 to the data generating process:

\[ \text{logit}(y) = -0.66 + 0.15x_1 + 0.25x_2 + 0.10b_1 + 0.30b_2\]

The simulation now gives us the desired target:

```
d1a <- defData(d1, varname = "y",
formula = "t(c(-0.66, ..coefs1)) %*% c(1, x1, x2, b1, b2)",
dist = "binary", link = "logit")
genData(500000, d1a)[, mean(y)]
```

`## [1] 0.4`

If we change the distribution of the covariates, so that \(x_1 \sim N(1, 1)\), \(x_2 \sim N(2, 1)\), \(b_1 \sim Bin(0.5)\), and \(b_2 \sim Bin(0.8)\), and the strength of the association of these covariates with the outcome so that

\[ \text{logit}(y) = 0.20x_1 + 0.35x_2 + 0.20b_1 + 0.45b_2,\]

the marginal proportion/prevalence (assuming no intercept term) also changes, going from 0.56 to 0.84:

```
coefs2 <- c(0.20, 0.35, 0.20, 0.45)
d2 <- defData(varname = "x1", formula = 1, variance = 1)
d2 <- defData(d2, varname = "x2", formula = 3, variance = 1)
d2 <- defData(d2, varname = "b1", formula = 0.5, dist = "binary")
d2 <- defData(d2, varname = "b2", formula = 0.8, dist = "binary")
d2a <- defData(d2, varname = "y",
formula = "t(..coefs2) %*% c(x1, x2, b1, b2)",
dist = "binary", link = "logit")
genData(500000, d2a)[, mean(y)]
```

`## [1] 0.84`

But under this new distribution, adding an intercept of -2.13 yields the desired target.

\[ \text{logit}(y) = -2.13 + 0.20x_1 + 0.35x_2 + 0.20b_1 + 0.45b_2 \]

```
d2a <- defData(d2, varname = "y",
formula = "t(c(-2.13, ..coefs2)) %*% c(1, x1, x2, b1, b2)",
dist = "binary", link = "logit")
genData(500000, d1a)[, mean(y)]
```

`## [1] 0.4`

Where did those two intercepts come from? The paper by Peter Austin describes an iterative bisection procedure that takes a distribution of covariates and a set of coefficients to identify the intercept coefficient that yields the target marginal proportion or prevalence.

The general idea of the algorithm is to try out series of different
intercepts in an intelligent way that ends up at the right spot. (If you
want the details for the algorithm, take a look at the paper.) The starting search range is pre-defined
(we’ve used -10 to 10 for the intercept), and we start with an value of
0 for the initial intercept and simulate a large data set (the paper
uses 1 million observations, but 100,000 seems to work just fine) and
record the population prevalence. If we’ve overshot the target
prevalence, we turn our attention to the range between -10 and 0, taking
the average, which is -5. Otherwise, we focus on the range between 0 and
10. We iterate this way, choosing the range we need to focus on and
setting the intercept at the mid-point (hence the name
*bisection*). The algorithm will converge pretty quickly on the
value of the intercept that gives the target population prevalence for
the underlying covariate distribution and coefficient assumptions.

In the current implementation in `simstudy`

, the intercept
is provided by a simple call to `logisticCoefs`

. Here are the
calls for the two sets of definitions in definition tables *d1*
and *d2*.

```
## B0 x1 x2 b1 b2
## -0.66 0.15 0.25 0.10 0.30
```

```
## B0 x1 x2 b1 b2
## -2.13 0.20 0.35 0.20 0.45
```

Just as the prevalence depends on the distribution of covariates and their association with the outcome, risk ratios comparing the outcome probabilities for two groups also depend on the additional covariates. The marginal risk ratio comparing treatment (\(A =1\) to control (\(A=0\)) (given the distribution of covariates) is

\[RR = \frac{P(y=1 | A = 1)}{P(y=1 | A =
0)}\] In the data generation process we use a log-odds ratio of
-0.40 (odds ratio of approximately 0.67) in both cases, but we get
different risk ratios (0.82 vs. 0.93), depending on the covariates
(defined in *d1* and *d2*).

```
d1a <- defData(d1, varname = "rx", formula = "1;1", dist = "trtAssign")
d1a <- defData(d1a, varname = "y",
formula = "t(c(-0.40, ..coefs1)) %*% c(rx, x1, x2, b1, b2)",
dist = "binary", link = "logit"
)
dd <- genData(500000, d1a)
dd[rx==1, mean(y)]/dd[rx==0, mean(y)]
```

`## [1] 0.82`

```
d2a <- defData(d2, varname = "rx", formula = "1;1", dist = "trtAssign")
d2a <- defData(d2a, varname = "y",
formula = "t(c(-0.40, ..coefs2)) %*% c(rx, x1, x2, b1, b2)",
dist = "binary", link = "logit"
)
dd <- genData(500000, d2a)
dd[rx==1, mean(y)]/dd[rx==0, mean(y)]
```

`## [1] 0.93`

By specifying both a population prevalence and a target risk ratio in
the call to `logisticCoefs`

, we can get the necessary
parameters. When specifying the target risk ratio, it is required to be
between 0 and 1/popPrev. A risk ratio cannot be negative, and the
probability of the outcome under treatment cannot exceed 1 (which will
happen if the risk ratio is greater than 1/popPrev).

```
## B0 A x1 x2 b1 b2
## -0.66 -0.26 0.15 0.25 0.10 0.30
```

If we use \(C_1\) in the data generation process, we will get a data set with the desired target prevalence and risk ratio:

```
d1a <- defData(d1, varname = "rx", formula = "1;1", dist = "trtAssign")
d1a <- defData(d1a, varname = "y",
formula = "t(..C1) %*% c(1, rx, x1, x2, b1, b2)",
dist = "binary", link = "logit"
)
dd <- genData(500000, d1a)
```

Here are the prevalence and risk ratio:

`## [1] 0.4`

`## [1] 0.86`

You can do the same for the second set of assumptions.

Risk differences have the same set of issues, and are handled in the same way. The risk difference is defined as

\[ RD = P(y=1 | A = 1) - P(y=1 | A = 0)\]

To get the coefficients related to a population prevalence of 0.40
and risk difference of -0.15 (so that the proportion in the exposure arm
is 0.25), we use the *rd* argument:

```
## B0 A x1 x2 b1 b2
## -0.66 -0.71 0.15 0.25 0.10 0.30
```

Again, using \(C_1\) in the data generation process, we will get a data set with the desired target prevalence and risk difference:

```
d1a <- defData(d1, varname = "rx", formula = "1;1", dist = "trtAssign")
d1a <- defData(d1a, varname = "y",
formula = "t(..C1) %*% c(1, rx, x1, x2, b1, b2)",
dist = "binary", link = "logit"
)
dd <- genData(500000, d1a)
dd[rx==0, mean(y)]
```

`## [1] 0.4`

`## [1] -0.15`

The AUC is another commonly used statistic to evaluate a logistic
model. We can use `logisticCoefs`

to find the parameters that
will allow us to generate data from a model with a specific AUC. To get
the coefficients related to a population prevalence of 0.40 and an AUC
of 0.85, we use the *auc* argument (which must be between 0.5 and
1):

```
## B0 x1 x2 b1 b2
## -1.99 0.85 1.41 0.56 1.69
```

Again, using \(C_1\) in the data
generation process, we will get a data set with the desired target
prevalence and the AUC (calculated here using the `lrm`

function in the `rms`

package:

```
d1a <- defData(d1, varname = "y",
formula = "t(..C1) %*% c(1, x1, x2, b1, b2)",
dist = "binary", link = "logit"
)
dd <- genData(500000, d1a)
dd[, mean(y)]
```

`## [1] 0.4`

```
## C
## 0.85
```

References:

Austin, Peter C. “The iterative bisection procedure: a useful tool for determining parameter values in data-generating processes in Monte Carlo simulations.” BMC Medical Research Methodology 23, no. 1 (2023): 1-10.