In this question the answerer goes into what the R quantile() function generally does, but I don't really feel satisfied that I understand the different possible modes of the function.

I've been using quantile() to find the nth percentile of a set of data, as it turns out it was suggested in the top answer here, but now that I'm looking into it further, I don't know how reliable that solution is.

I'm using the default type - type 7 - now, but the documentation for the function states that type 8 is the "recommended" type. Recommended for what? What is it finding, and how is that superior? Is it better for finding the percentiles?

I generally do not understand the difference between what exactly the different quantile() modes are finding. To make things even more confusing, my coworker ran a test of the two types and the percentiles produced by type 8 were not bijective, which would be impossible -- but I haven't been able to replicate his results, so I don't know how type 8 could or could not produce non-bijective percentiles.

Any explanation would be appreciated.

  • 3
    $\begingroup$ The documentation provides a reference. Read it! $\endgroup$ – Roland May 20 '16 at 7:04

Hyndman and Fan (1996) (also here) described 9 algorithms to calculate a quantile.

Sample quantiles of type i are defined by Q[i](p) = (1 - gamma) x[j] + gamma x[j+1]

where i in [1,9], p in [(j-m)/n,(j-m+1)/ n] , x[j] is the j-th order statistic, n is the sample size, and m is a constant determined by the sample quantile type. Here gamma depends on the fractional part of g = np+m-j.

For the continuous sample quantile types (4 through 9), the sample quantiles can be obtained by linear interpolation between the k-th order statistic and p(k):

p(k) = (k - alpha) / (n - alpha - beta + 1)

where alpha and beta are constants determined by the type. Further, m = alpha + p(1 - alpha - beta), and gamma = g.

About the two types you are using (7 and 8, the default):

  • Type 7 (used by S)

p(k) = (k - 1) / (n - 1) In this case, p(k) = mode[F(x[k])].

  • Type 8

p(k) = (k - 1/3) / (n + 1/3)

Then p(k) =~ median[F(x[k])]. The resulting quantile estimates are approximately median-unbiased regardless of the distribution of 'x'.

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