adaptmean {upndown}R Documentation

Up-and-Down averaging estimate with adaptive starting-point

Description

A dose-averaging estimate based on a concept from Oron (2007). Provides an alternative to reversal-based averaging.

Usage

adaptmean(x, maxExclude = 1/3, before = FALSE, full = FALSE)

Arguments

x

numeric vector: sequence of administered doses, treatments, stimuli, etc.

maxExclude

a fraction in 0,1 indicating the maximum initial fraction of the vector x to exclude from averaging, in case the algorithm-identified starting point occurs too late in the experiment. Default 1/3.

before

logical: whether to start the averaging one step earlier than the starting reversal point. Default FALSE, and ignored when all=FALSE.

full

logical: should more detailed information be returned, or only the estimate? (default FALSE)

Details

Historically, most up-and-down studies have used dose-averaging estimates. Many of them focus on reversal points either as anchor/cutoff points – points where the averaging begins – or as the only doses to use in the estimate. Excluding doses before the anchor/cutoff is done in order to mitigate the bias due to the arbitrary location of the starting dose. The extent of excluded sample depends on the distance between the starting dose and the up-and-down balance point, as well as the random-walk vagaries of an individual experimental run.

Oron (2007) showed that using only reversals and skipping other doses is generally a bad idea, and also noted that a reversal anchor point is not directly tied to the conceptual motivation for having an anchor/cutoff point.

In practice, some "lucky" experiments might not need any exclusion at all (because they started right at the balance point), while others might need to exclude dozens of observations. Reversals do not capture this variability well.

The estimation method coded in adaptmean() works from a different principle. It identifies the first crossing point: the first point at which the dose is "on the other side" from the starting point, compared with the average of all remaining doses. The average of all remaining doses is used as a proxy to the (unobservable) balance point. This approach is far closer to capturing the dynamics described above, and indeed performs well in comparative simulations (Oron et al. 2022, Supplement).

Interestingly, unlike other methods adaptmean() does not require the experiment's binary responses as input; only the dose-allocation sequence.

The reason adaptmean() has not been further developed nor published, is that like all dose-averaging estimators, at present there doesn't seem to be a reliable confidence interval to accompany any of them.

For UDD target estimation we recommend using centered isotonic regression, a more robust method available together with a confidence interval via udest, an up-and-down adapted wrapper to cir::quickInverse(). See Oron et al. 2022 (both article and supplement) for further information, as well as the cir package vignette.

Value

The point estimate

Author(s)

Assaf P. Oron <assaf.oron.at.gmail.com>

References

See Also

Examples

#'  **An up-and-down experiment that has generated some controversy**
#'  
#' Van Elstraete, AC et al. The Median Effective Dose of Preemptive Gabapentin 
#'      on Postoperative Morphine Consumption After Posterior Lumbar Spinal Fusion. 
#'      *Anesthesia & Analgesia* 2008, 106: 305-308.


# It was a classical median-finding up-and-down study.

doses = c(4:7, 6:13, 12:19, 18:21, 20, 19:23, 22, 21:23, 22:19, 20:23, 
          22:24, 23, 22, 23, 22:25, 24:22, rep(23:24,2), 23, 22)
# With U&D, responses (except the last one) can be read off the doses:
responses = c( (1 - sign(diff(doses)))/2, 0 )


### Let us plot the dose-allocation time series.

# Saving current settings as now required by the CRAN powers-that-be :0
op <- par(no.readonly = TRUE)

par(mar=c(4,4,4,1), mgp=c(2.5,0.8,0), cex.axis = 0.7, las = 1)
udplot(doses, responses, main='Van Elstraete et al. 2008 Study', 
       xtitle = "Patient Number", ytitle = 'Gabapentin (mg/kg)') 


#' Overlay the ED50 reported in the article (21.7 mg/kg):
abline(h = 21.7)

#' The authors cite a little-known 1991 article by Dixon as the method source.
#' However, in their author rejoinder they claim to have used the Dixon-Mood (1948) estimate.


# Our package does include the Dixon-Mood point estimate.
#  (w/o the CIs, because we do not endorse this estimation approach)
# Does it reproduce the article estimate?
dixonmood(doses, responses)

# Not at all! Let us overlay this one in red
abline(h = dixonmood(doses, responses), col=2)

# We have found that many articles claiming to use Dixon-Mood (or Dixon-Massey) actually
# Do something else. For example, in this article they report that 
#   "it is necessary to reject sequences with three to six identical results".
# Nothing like this appears in the original Dixon-Mood article, where the estimation method
#   involves identifying the less-common response (either 0 or 1), and using only x values
#   associated with these responses; obviating the need to exclude specific sequences.
#
# More generally, these historical estimates have long passed their expiry dates. 
#   Their foundation is not nearly as solid as, e.g., linear regression, 
#      and it's time to stop using them.

# That said, our package does offer two more types of dose-averaging estimates.
# Both are able to take advantage of the "n+1" dose-allocation, which is determined by
#    the last dose and response:
n = length(doses)
dosePlus1 = doses[n] + ifelse(responses[n]==0, 1, -1)
reversmean(c(doses, dosePlus1), responses)
# Interestingly, in this particular case the answer is very similar to the Dixon-Mood estimate.

# The `reversmean()` default averages all doses from the 3rd reversal point onwards.
# By the way, at what point did the third reversal happen? 
#     It'll be the 3rd number in this vector:
reversals(responses)

# Far more commonly in literature, particularly in sensory studies, 
#   one encounters the 1960s-era approach (led by Wetherill) of taking *only doses  
#   at reversal points, usually starting from the first one. `reversmean()` can do that too:
wetherill = reversmean(c(doses, dosePlus1), responses, all = FALSE, rstart = 1)
wetherill
# This one gives an even lower result than the previous ones.
abline(h = wetherill, col = 3)

# There's another approach to dose-averaging, although it is not in use anywhere that we know of.
# It does not require the y values at all. The underlying assumption is that the dose 
#   sequence has done enough meandering around the true balance point, to provide information
#   about where (approximately) the starting-dose effect is neutralized.
adaptmean(c(doses, dosePlus1))
# Again a bit curiously, this relatively recent approach gives a result similar to what
#   the authors reported (but not similar to the original Dixon-Mood).
# This is not too surprising, since here `adaptmean()` excludes the first one-third of doses,
#   which is approximately what happened if indeed the authors excluded all those long dose-increase
#   sequences at the start.

# All this shows how dicey dose-averaging, at face value a simple and effective method, can become.
# The sample size here is rather large for up-and-down studies, and yet because of the unlucky
#    choice of starting point (which in many studies, due to safety concerns cannot be evaded)
#    there is really no good option of which observations to exclude.

# This is one reason why we strongly recommend using Centered Isotonic Regression as default:
defest = udest(doses, responses, target = 0.5)
abline(h = defest$point, col = 'purple')
# For this dataset, it is the highest of all the estimates.

legend('bottomright', col = c(1:3, 'purple'), 
       legend = c("Article's estimate", 'Dixon-Mood', 'Reversals (Wetherill)', 'Standard (CIR)'), 
       lty = 1, bty='n', cex = 0.8)

par(op) # Back to business as usual ;)
[Package upndown version 0.1.0 Index]