Posts / Efficiency of the winsorized and trimmed Harrell-Davis quantile estimators

Update: this blog post is a part of research that aimed to build a statistically efficient and robust quantile estimator. A paper with final results is available in Communications in Statistics - Simulation and Computation (DOI: 10.1080/03610918.2022.2050396). A preprint is available on arXiv: arXiv:2111.11776 [stat.ME]. Some information in this blog post can be obsolete: please, use the official paper as the primary reference.

In previous posts, I suggested two modifications of the Harrell-Davis quantile estimator: winsorized and trimmed. Both modifications have a higher level of robustness in comparison to the original estimator. Also, I discussed the efficiency of the Harrell-Davis quantile estimator. In this post, I’m going to continue numerical simulation and estimate the efficiency of the winsorized and trimmed modifications.

Simulation design

The relative efficiency value depends on five parameters:

Our target quantile estimator is the Harrell-Davis (HD) quantile estimator ( harrell1982).

$$ Q_{HD}(p) = \sum_{i=1}^{n} W_{i} \cdot x_{(i)}, \quad W_{i} = I_{i/n}(a, b) - I_{(i-1)/n}(a, b), \quad a = p(n+1),\; b = (1-p)(n+1) $$

where $I_t(a, b)$ denotes the regularized incomplete beta function, $x_{(i)}$ is the $i^\textrm{th}$ order statistics. Also, we consider winsorized (WHD) and trimmed (THD) modifications of HD quantile estimator. In this simulation, we use 0.05 as the trimming percentage.

The conventional baseline quantile estimator in such simulations is the traditional quantile estimator that is defined as a linear combination of two subsequent order statistics. To be more specific, we are going to use the Type 7 quantile estimator from the Hyndman-Fan classification or HF7 ( hyndman1996). It can be expressed as follows (assuming one-based indexing):

$$ Q_{HF7}(p) = x_{(\lfloor h \rfloor)}+(h-\lfloor h \rfloor)(x_{(\lfloor h \rfloor+1)})-x_{(\lfloor h \rfloor)},\quad h = (n-1)p+1. $$

Thus, we are going to estimate the relative efficiency of HD, WHD (0.05), THD (0.05) quantile estimators comparing to the traditional quantile estimator HF7. For the $p^\textrm{th}$ quantile, the relative efficiency can be calculated as the ratio of the estimator mean squared errors ($\textrm{MSE}$):

$$ \textrm{Efficiency}(p) = \dfrac{\textrm{MSE}(Q_{HF7}, p)}{\textrm{MSE}(Q_{HD}, p)} = \dfrac{\operatorname{E}[(Q_{HF7}(p) - \theta(p))^2]}{\operatorname{E}[(Q_{HD}(p) - \theta(p))^2]} $$

where $\theta(p)$ is the true value of the $p^\textrm{th}$ quantile. The $\textrm{MSE}$ value depends on the sample size $n$, so it should be calculated independently for each sample size value.

Finally, we should choose the distributions for sample generation. I decided to choose 4 light-tailed distributions and 4 heavy-tailed distributions

distributiondescription
Beta(2,10)Beta distribution with a=2, b=10
U(0,1)Uniform distribution on [0;1]
N(0,1^2)Normal distribution with mu=0, sigma=1
Weibull(1,2)Weibull distribution with scale=1, shape=2
Cauchy(0,1)Cauchy distribution with location=0, scale=1
Pareto(1, 0.5)Pareto distribution with xm=1, alpha=0.5
LogNormal(0,3^2)Log-normal distribution with mu=0, sigma=3
Exp(1) + Outliers95% of exponential distribution with rate=1 and 5% of uniform distribution on [0;10000]

Here are the probability density functions of these distributions:

For each distribution, we are going to do the following:

Here are the results of the simulation:

Conclusion

Based on the above simulation, we could draw the following conclusions:

References


References (4)