A little off topic: data.frame members [R for BE/BA]

posted by ElMaestro  – Belgium?, 2018-01-22 23:49  – Posting: # 18254
Views: 3,938

Hi all,
:ok:
there is something about R internals that I don 't understand and I can best exemplify it using some code from power.TOST but my question really isn't about anything in that package.

Imagine I do this in R with powerTOST:

sampleN.RSABE(CV=0.3, theta0=0.95)
Nice.
Now, assume I don't want all that intermediate mumbojumbo, and let us for the time being completely forget the print/verbose/details option so I just want the sample size itself as a numeric.

If I inspect the code I see the last few lines:

    res <- data.frame(design = design, alpha = alpha, CVwT = CVwT,
        CVwR = CVwR, theta0 = theta0, theta1 = theta1, theta2 = theta2,
        n = n, power = pwr, targetpower = targetpower, nlast = nlast)
    names(res) <- c("Design", "alpha", "CVwT", "CVwR", "theta0",
        "theta1", "theta2", "Sample size", "Achieved power",
        "Target power", "nlast")
    if (print | details)
        return(invisible(res))
    else return(res)


...and therefore I can simply do this:
sampleN.RSABE(CV=0.3, theta0=0.95)$n
(the n column is baptised as "Sample size" but is still accessible via $n)

If I look at the corresponding code snippet from sampleN.TOST we have:
     res <- data.frame(design = design, alpha = alpha, CV = CV,
        theta0 = theta0, theta1 = theta1, theta2 = theta2, n = n,
        power = pow, targetpower = targetpower)
    names(res) <- c("Design", "alpha", "CV", "theta0", "theta1",
        "theta2", "Sample size", "Achieved power", "Target power")
    if (print)
        return(invisible(res))
    else return(res)


But if I go
sampleN.TOST(CV=0.3, theta0=0.95)$n
then I get NULL.

What determines that difference in behaviour? Why does the res data.frame from both the two functions not hold an extractable $n ?

Note, this really is solely about R behaviour and R function return/internals (if I want the sample sizes I can also easily get them via [1,7] or [1,8] from the sample size object returned; all this isn't really what this question is about).:-)

I could be wrong, but...

Best regards,
ElMaestro

"Pass or fail" (D. Potvin et al., 2008)

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