Case study 1 deals with a simple setting, namely, a clinical trial with two treatment arms (experimental treatment versus placebo) and a single endpoint. Power calculations can be performed analytically in this setting. Specifically, closed-form expressions for the power function can be derived using the central limit theorem or other approximations.

Several distribution will be illustrated in this case study:

Normally distributed endpoint

Suppose that a sponsor is designing a Phase III clinical trial in patients with pulmonary arterial hypertension (PAH). The efficacy of experimental treatments for PAH is commonly evaluated using a six-minute walk test and the primary endpoint is defined as the change from baseline to the end of the 16-week treatment period in the six-minute walk distance.

Define a Data Model

The first step is to initialize the data model:

After the initialization, components of the data model can be added to the DataModel object incrementally using the + operator.

The change from baseline in the six-minute walk distance is assumed to follow a normal distribution. The distribution of the primary endpoint is defined in the OutcomeDist object:

The sponsor would like to perform power evaluation over a broad range of sample sizes in each treatment arm:

As a side note, the seq function can be used to compactly define sample sizes in a data model:

The sponsor is interested in performing power calculations under two treatment effect scenarios (standard and optimistic scenarios). Under these scenarios, the experimental treatment is expected to improve the six-minute walk distance by 40 or 50 meters compared to placebo, respectively, with the common standard deviation of 70 meters.

Scenario Mean (SD)
Placebo Treatment
Standard 0 (70) 40 (70)
Optimistic 0 (70) 50 (70)

Therefore, the mean change in the placebo arm is set to μ = 0 and the mean changes in the six-minute walk distance in the experimental arm are set to μ = 40 (standard scenario) or μ = 50 (optimistic scenario). The common standard deviation is σ = 70.

Note that the mean and standard deviation are explicitly identified in each list. This is done mainly for the user’s convenience.

After having defined the outcome parameters for each sample, two Sample objects that define the two treatment arms in this trial can be created and added to the DataModel object:

Define an Analysis Model

Just like the data model, the analysis model needs to be initialized as follows:

Only one significance test is planned to be carried out in the PAH clinical trial (treatment versus placebo). The treatment effect will be assessed using the one-sided two-sample t-test:

According to the specifications, the two-sample t-test will be applied to Sample 1 (Placebo) and Sample 2 (Treatment). These sample IDs come from the data model defied earlier. As explained in the manual, see Analysis Model, the sample order is determined by the expected direction of the treatment effect. In this case, an increase in the six-minute walk distance indicates a beneficial effect and a numerically larger value of the primary endpoint is expected in Sample 2 (Treatment) compared to Sample 1 (Placebo). This implies that the list of samples to be passed to the t-test should include Sample 1 followed by Sample 2. It is of note that from version 1.0.6, it is possible to specify an option to indicate if a larger numeric values is expected in the Sample 2 (larger = TRUE) or in Sample 1 (larger = FALSE). By default, this argument is set to TRUE.

To illustrate the use of the Statistic object, the mean change in the six-minute walk distance in the treatment arm can be computed using the MeanStat statistic:

Define an Evaluation Model

The data and analysis models specified above collectively define the Clinical Scenarios to be examined in the PAH clinical trial. The scenarios are evaluated using success criteria or metrics that are aligned with the clinical objectives of the trial. In this case it is most appropriate to use regular power or, more formally, marginal power. This success criterion is specified in the evaluation model.

First of all, the evaluation model must be initialized:

Secondly, the success criterion of interest (marginal power) is defined using the Criterion object:

The tests argument lists the IDs of the tests (defined in the analysis model) to which the criterion is applied (note that more than one test can be specified). The test IDs link the evaluation model with the corresponding analysis model. In this particular case, marginal power will be computed for the t-test that compares the mean change in the six-minute walk distance in the placebo and treatment arms (Placebo vs treatment).

In order to compute the average value of the mean statistic specified in the analysis model (i.e., the mean change in the six-minute walk distance in the treatment arm) over the simulation runs, another Criterion object needs to be added:

The statistics argument of this Criterion object lists the ID of the statistic (defined in the analysis model) to which this metric is applied (e.g., Mean Treatment).

Perform Clinical Scenario Evaluation

After the clinical scenarios (data and analysis models) and evaluation model have been defined, the user is ready to evaluate the success criteria specified in the evaluation model by calling the CSE function.

To accomplish this, the simulation parameters need to be defined in a SimParameters object:

The function call for CSE specifies the individual components of Clinical Scenario Evaluation in this case study as well as the simulation parameters:

The simulation results are saved in an CSE object (case.study1.results). This object contains complete information about this particular evaluation, including the data, analysis and evaluation models specified by the user. The most important component of this object is the data frame contained in the list named simulation.results (case.study1.results\$simulation.results). This data frame includes the values of the success criteria and metrics defined in the evaluation model.

Summarize the Simulation Results

Summary of simulation results in R console

To facilitate the review of the simulation results produced by the CSE function, the user can invoke the summary function. This function displays the data frame containing the simulation results in the R console:

If the user is interested in generate graphical summaries of the simulation results (using the the ggplot2 package or other packages), this data frame can also be saved to an object:

General a Simulation Report

Presentation Model

A very useful feature of the Mediana package is generation of a Microsoft Word-based report to provide a summary of Clinical Scenario Evaluation Report.

To generate a simulation report, the user needs to define a presentation model by creating a PresentationModel object. This object must be initialized as follows:

Project information can be added to the presentation model using the Project object:

The user can easily customize the simulation report by defining report sections and specifying properties of summary tables in the report. The code shown below creates a separate section within the report for each set of outcome parameters (using the Section object) and sets the sorting option for the summary tables (using the Table object). The tables will be sorted by the sample size. Further, in order to define descriptive labels for the outcome parameter scenarios and sample size scenarios, the CustomLabel object needs to be used:

Report generation

Once the presentation model has been defined, the simulation report is ready to be generated using the GenerateReport function:

Click on the icons below to download the R code used in this case study and Clinical Scenario Evaluation Report generated by the GenerateReport function:

Binary endpoint

Consider a Phase III clinical trial for the treatment of rheumatoid arthritis (RA). The primary endpoint is the response rate based on the American College of Rheumatology (ACR) definition of improvement. The trial’s sponsor in interested in performing power calculations using the treatment effect assumptions listed in the table below:

Outcome parameter set Response rate
Placebo Treatment
Pessimistic 30% 50%
Standard 30% 55%
Optimistic 30% 60%

Define a Data Model

The three outcome parameter sets displayed in the table are combined with four sample size sets (SampleSize(c(80, 90, 100, 110))) and the distribution of the primary endpoint (OutcomeDist(outcome.dist = "BinomDist")) is specified in the DataModel object case.study1.data.model:

Define an Analysis Model

The analysis model uses a standard two-sample test for comparing proportions (method = "PropTest") to assess the treatment effect in this clinical trial example:

Define an Evaluation Model

Power evaluations are easily performed in this clinical trial example using the same evaluation model utilized in the case of a normally distributed endpoint, i.e., evaluations rely on marginal power:

Click on the icons below to download the R code used in this case study and report that summarizes the results of Clinical Scenario Evaluation:

An extension of this clinical trial example is provided in Case study 5. The extension deals with a more complex setting involving several trial endpoints and multiple treatment arms.

Survival-type endpoint

If the trial’s primary objective is formulated in terms of analyzing the time to a clinically important event (progression or death in an oncology setting), data and analysis models can be set up based on an exponential distribution and the log-rank test.

As an illustration, consider a Phase III trial which will be conducted to evaluate the efficacy of a new treatment for metastatic colorectal cancer (MCC). Patients will be randomized in a 2:1 ratio to an experimental treatment or placebo (in addition to best supportive care).

The trial’s primary objective is to assess the effect of the experimental treatment on progression-free survival (PFS).

Define a Data Model

A single treatment effect scenario is considered in this clinical trial example. Specifically, the median time to progression is assumed to be:

• Placebo : t0 = 6 months,

• Treatment: t1 = 9 months.

Under an exponential distribution assumption (which is specified using the ExpoDist distribution), the median times correspond to the following hazard rates:

• λ0 = log(2)/t0 = 0.116,

• λ1 = log(2)/t1 = 0.077,

and the resulting hazard ratio (HR) is 0.077/0.116 = 0.67.

It is important to note that, if no censoring mechanisms are specified in a data model with a time-to-event endpoint, all patients will reach the endpoint of interest (e.g., progression) and thus the number of patients will be equal to the number of events. Using this property, power calculations can be performed using either the Event object or SampleSize object. For the purpose of illustration, the Event object will be used in this example.

To define a data model in the MCC clinical trial, the total event count in the trial is assumed to range between 270 and 300. Since the trial’s design is not balanced, the randomization ratio needs to be specified in the Event object:

It is worth noting that the primary endpoint’s type (i.e., theoutcome.type argument in the OutcomeDist object) is not specified. By default, the outcome type is set to fixed, which means that a design with a fixed follow-up is assumed even though the primary endpoint in this clinical trial is clearly a time-to-event endpoint. This is due to the fact that, as was explained earlier in this case study, there is no censoring in this design and all patients are followed until the event of interest is observed. It is easy to verify that the same results are obtained if the outcome type is set to event.

Define an Analysis Model

The analysis model in this clinical trial is very similar to the analysis models defined in the case studies with normal and binomial outcome variables. The only difference is the choice of the statistical method utilized in the primary analysis (method = "LogrankTest"):

To illustrate the specification of a Statistic object, the hazard ratio will be computed using the Cox method. This can be accomplished by adding a Statistic object to the AnalysisModel such presented below.

Define an Evaluation Model

An evaluation model identical to that used earlier in the case studies with normal and binomial distribution can be applied to compute the power function at the selected event counts. Moreover, the average hazard ratio accross the simulations will be computed.

Click on the icons below to download the R code used in this case study and report that summarizes the results of Clinical Scenario Evaluation:

Survival-type endpoint (with censoring)

The power calculations presented in the previous case study assume an idealized setting where each patient is followed until the event of interest (e.g., progression) is observed. In this case, the sample size (number of patients) in each treatment arm is equal to the number of events. In reality, events are often censored and a sponsor is generally interested in determining the number of patients to be recruited in order to ensure a target number of events, which translates into desirable power.

The Mediana package can be used to perform power calculations in event-driven trials in the presence of censoring. This is accomplished by setting up design parameters such as the length of the enrollment and follow-up periods in a data model using a Design object.

In general, even though closed-form solutions have been derived for sample size calculations in event-driven designs, the available approaches force clinical trial researchers to make a variety of simplifying assumptions, e.g., assumptions on the enrollment distribution are commonly made, see, for example, Julious (2009, Chapter 15). A general simulation-based approach to power and sample size calculations implemented in the Mediana package enables clinical trial sponsors to remove these artificial restrictions and examine a very broad set of plausible design parameters.

Define a Data Model

Suppose, for example, that a standard design with a variable follow-up will be used in the MCC trial introduced in the previous case study. The total study duration will be 21 months, which includes a 9-month enrollment (accrual) period and a minimum follow-up of 12 months. The patients are assumed to be recruited at a uniform rate. The set of design parameters also includes the dropout distribution and its parameters. In this clinical trial, the dropout distribution is exponential with a rate determined from historical data. These design parameters are specified in a Design object:

Finally, the primary endpoint’s type is set to event in the OutcomeDist object to indicate that a variable follow-up will be utilized in this clinical trial.

The complete data model in this case study is defined as follows:

Define an Analysis Model

Since the number of events has been fixed in this clinical trial example and some patients will not reach the event of interest, it will be important to estimate the number of patients required to accrue the required number of events. In the Mediana package, this can be accomplished by specifying a descriptive statistic named PatientCountStat (this statistic needs to be specified in a Statistic object). Another descriptive statistic that would be of interest is the event count in each sample. To compute this statistic, EventCountStat needs to be included in a Statistic object.

Define an Evaluation Model

In order to compute the average values of the two statistics (PatientCountStat and EventCountStat) in each sample over the simulation runs, two Criterion objects need to be specified, in addition to the Criterion object defined to obtain marginal power. The IDs of the corresponding Statistic objects will be included in the statistics argument of the two Criterion objects:

Click on the icons below to download the R code used in this case study and report that summarizes the results of Clinical Scenario Evaluation:

Count-type endpoint

The last clinical trial example within Case study 1 deals with a Phase III clinical trial in patients with relapsing-remitting multiple sclerosis (RRMS). The trial aims at assessing the safety and efficacy of a single dose of a novel treatment compared to placebo. The primary endpoint is the number of new gadolinium enhancing lesions seen during a 6-month period on monthly MRIs of the brain and a smaller number indicates treatment benefit. The distribution of such endpoints has been widely studied in the literature and Sormani et al. (1999a, 1999b) showed that a negative binomial distribution provides a fairly good fit.

The table below gives the expected treatment effect in the experimental treatment and placebo arms (note that the negative binomial distribution is parameterized using the mean rather than the probability of success in each trial).

Treatment Arm Mean number of new lesions Dispersion parameter
Placebo 13 0.5
Treatment 7.8 0.5

The corresponding treatment effect, i.e., the relative reduction in the mean number of new lesions counts, is 100 * (13 − 7.8)/13 = 40%. The assumptions in the table define a single outcome parameter set.

Define a Data Model

The OutcomeDist object defines the distribution of the trial endpoint (NegBinomDist). Further, a balanced design is utilized in this clinical trial and the range of sample sizes is defined in the SampleSize object (it is convenient to do this using the seq function). The Sample object includes the parameters required by the negative binomial distribution (dispersion and mean).

Define an Analysis Model

The treatment effect will be assessed in this clinical trial example using a negative binomial generalized linear model (NBGLM). In the Mediana package, the corresponding test is carrying out using the GLMNegBinomTest method which is specified in the Test object. It should be noted that as a smaller value indicates a treatment benefit, the first sample defined in the samples argument must be Treatment.

Alternatively, from version 1.0.6, it is possible to specify the argument lower in the parameters of the method. If set to FALSE a numerically lower value is expected in Sample 2.

Define an Evaluation Model

The objective of this clinical trial is identical to that of the clinical trials presented earlier on this page, i.e., evaluation will be based on marginal power of the primary endpoint test. As a consequence, the same evaluation model can be applied.