Agreement Analysis for Continuous Data with Repeated Measurements

agreement.metrics <- function(dat)
{
    ## Purpose: 
    ##   Calculate agreement metrics. 
    ## 
    ## Author: Feiming Chen
    ## 
    ## Arguments: 
    ##   - dat : a data frame with the following variable names and with no NA values
    ##           subject - subject identifier
    ##           reference - continuous response from reference
    ##           device - continuous response from device
    ##           
    ## Return: 
    ##   - a list of agreement metrics (MAE, MAE%, RMSE, LOA, Bias, Average Correlation). 
    ## 
    ## ________________________________________________

    reference.mean <- mean(dat$reference)
    dat$difference <- dat$device - dat$reference

    ## Bland-Altman Analysis with Repeated Measures
    BA.analysis <- bland.altman.repeated.vary(difference ~ factor(subject), dat, silent = TRUE)

    ## For MAE and RMSE
    MAE <- mean(abs(dat$difference))    # Mean Absolute Error
    MAE.pct <- MAE / reference.mean     
    RMSE <- sqrt(mean(dat$difference^2)) # Root Mean Squared Error

    ## For Correlation: calculate patient-level summary metrics from repeated measures, then average across patients. 
    ds <- split(dat, dat$subject)
    sapply(ds, function(d) cor(d$reference, d$device, method = "pearson")) -> ds2
    Correlation <- mean(ds2)

    c(ref.mean = reference.mean,
      MAE = MAE,
      MAE.pct = MAE.pct,
      RMSE = RMSE,
      Correlation = Correlation, 
      Bias = BA.analysis$m,
      SD = BA.analysis$s, 
      LoA.L = BA.analysis$loa[1],
      LoA.U = BA.analysis$loa[2],
      SD0 = BA.analysis$s0,             # no adjustment for repeated measurements
      LoA.L0 = BA.analysis$loa0[1],     # no adjustment for repeated measurements
      LoA.U0 = BA.analysis$loa0[2])     # no adjustment for repeated measurements

}
if (F) {
    agreement.metrics(data.frame(subject = rep(1:30, 10), reference = 5 + rnorm(300), device = 6 + rnorm(300)))
}

agreement.analysis <- function(dat, boot.size = 2000)
{
    ## Purpose: 
    ##   Calculate agreement metrics (MAE, MAE%, RMSE, LOA, Bias, Average Correlation)
    ##   with their CI's, even in the presence of repeated measurements. 
    ## 
    ## Author: Feiming Chen
    ## 
    ## Arguments: 
    ##   - dat : a data frame with the following variable names
    ##           subject - subject identifier
    ##           reference - continuous response from reference
    ##           device - continuous response from device
    ##   - boot.size : bootstrap sample size. 
    ## 
    ## Return: 
    ##   - Point estimates and 95% CI's for a list of agreement metrics (MAE, MAE%, RMSE, LOA, Bias, Average Correlation). 
    ## 
    ## ________________________________________________


    id <- dat$subject
    unique.ID <- unique(id)
    N <- length(unique.ID)
    cat("Number of Unique Patients =", N, "\n")

    K <- nrow(dat) / N 
    cat("Total number of repeated measurements =", nrow(dat), "\n")
    cat("Average number of repeated measurements per patient =", K, "\n")

    ## Point Estimates
    est <- agreement.metrics(dat)
    cat("\nPoint Estimates of Agreement Metrics:\n")
    print(round(est, 4))

    ## 2000 Bootstrap Samples
    cat("\nBootstrap Sample Size =", boot.size, "\n")
    set.seed(1)
    replicate(n = boot.size, {
        s <- sample(unique.ID, replace = TRUE)  # a bootstrap sample of patient ID's
        ## find all rows with the ID's in the bootstrap sample of ID's 
        v <- c()
        for (j in s) v <- c(v, which(id == j)) # a new bootstrap sample
        agreement.metrics(dat[v,])
    }) -> est.boot

    CI <- apply(est.boot, 1, function(x) quantile(x, c(0.025, 0.975)))
    cat("95% Bootstrap Percentile Confidence Intervals of Agreement Metrics:\n")
    print(round(CI, 4))

    invisible(list(est = est, CI = CI))}

if (F) {                                # Unit Test
    ## ans <- agreement.analysis(dat, boot.size = 10)
}