Define the enrollment rate of subjects for a study as following a piecewise exponential distribution.

## Details

The `duration`

are ordered piecewise for a duration equal to
\(t_i - t_{i-1}\), where \(0 = t_0 < t_i < \cdots < t_M = \infty\).
The enrollment rates are defined in each duration with the same length.

For a study with multiple strata, different duration and rates can be specified in each stratum.

## Examples

```
# Define enroll rate without stratum
define_enroll_rate(
duration = c(2, 2, 10),
rate = c(3, 6, 9)
)
#> # A tibble: 3 × 3
#> stratum duration rate
#> <chr> <dbl> <dbl>
#> 1 All 2 3
#> 2 All 2 6
#> 3 All 10 9
# Define enroll rate with stratum
define_enroll_rate(
duration = rep(c(2, 2, 2, 18), 3),
rate = c((1:4) / 3, (1:4) / 2, (1:4) / 6),
stratum = c(array("High", 4), array("Moderate", 4), array("Low", 4))
)
#> # A tibble: 12 × 3
#> stratum duration rate
#> <chr> <dbl> <dbl>
#> 1 High 2 0.333
#> 2 High 2 0.667
#> 3 High 2 1
#> 4 High 18 1.33
#> 5 Moderate 2 0.5
#> 6 Moderate 2 1
#> 7 Moderate 2 1.5
#> 8 Moderate 18 2
#> 9 Low 2 0.167
#> 10 Low 2 0.333
#> 11 Low 2 0.5
#> 12 Low 18 0.667
```