npksum {np} | R Documentation

## Kernel Sums with Mixed Data Types

### Description

`npksum` computes kernel sums on evaluation data, given a set of training data, data to be weighted (optional), and a bandwidth specification (any bandwidth object).

### Usage

    npksum(...)

    ## S3 method for class 'formula'
    npksum(formula, data, newdata, subset, na.action, ...)

    ## Default S3 method:
    npksum(bws,
           txdat = stop("training data 'txdat' missing"),
           tydat = NULL,
           exdat = NULL,
           weights = NULL,
           leave.one.out = FALSE,
           kernel.pow = 1.0,
           bandwidth.divide = FALSE,
           operator = names(ALL_OPERATORS),
           permutation.operator = names(PERMUTATION_OPERATORS),
           compute.score = FALSE,
           compute.ocg = FALSE,
           return.kernel.weights = FALSE,
           ...)

    ## S3 method for class 'numeric'
    npksum(bws,
           txdat = stop("training data 'txdat' missing"),
           tydat,
           exdat,
           weights,
           leave.one.out,
           kernel.pow,
           bandwidth.divide,
           operator,
           permutation.operator,
           compute.score,
           compute.ocg,
           return.kernel.weights,
           ...)

### Arguments

`formula` | a symbolic description of variables on which the sum is to be performed. The details of constructing a formula are described below.
`data` | an optional data frame, list or environment (or object coercible to a data frame by `as.data.frame`) containing the variables in the model. If not found in data, the variables are taken from `environment(formula)`, typically the environment from which the function is called.
`newdata` | An optional data frame in which to look for evaluation data. If omitted, `data` is used.
`subset` | an optional vector specifying a subset of observations to be used.
`na.action` | a function which indicates what should happen when the data contain `NA`s. The default is set by the `na.action` setting of options, and is `na.fail` if that is unset. The (recommended) default is `na.omit`.
`...` | additional arguments supplied to specify the parameters to the `default` S3 method, which is called during estimation.
`txdat` | a p-variate data frame of sample realizations (training data) used to compute the sum.
`tydat` | a numeric vector of data to be weighted. The ith kernel weight is applied to the ith element. Defaults to `1`.
`exdat` | a p-variate data frame of sum evaluation points (if omitted, defaults to the training data itself).
`bws` | a bandwidth specification. This can be set as any suitable bandwidth object returned from a bandwidth-generating function, or a numeric vector.
`weights` | a n by q matrix of weights which can optionally be applied to `tydat` in the sum. See details.
`leave.one.out` | a logical value to specify whether or not to compute the leave one out sums. Will not work if `exdat` is specified. Defaults to `FALSE`.
`kernel.pow` | an integer specifying the power to which the kernels will be raised in the sum. Defaults to `1`.
`bandwidth.divide` | a logical specifying whether or not to divide continuous kernel weights by their bandwidths. Use this with nearest-neighbor methods. Defaults to `FALSE`.
`operator` | a string specifying whether the `normal`, `convolution`, `derivative`, or `integral` kernels are to be used. Operators scale results by factors of h or 1/h where appropriate. Defaults to `normal` and applies to all elements in a multivariate object. See details.

`permutation.operator`

a string which can have a value of `none`, `normal`, `derivative`, or `integral`. If set to something other than `none` (the default), then a separate result will be returned for each term in the product kernel, with the operator applied to that term. Permutation operators scale results by factors of h or 1/h where appropriate. This is useful for computing gradients, for example.

`compute.score` | a logical specifying whether or not to return the score (the ‘grad h’ terms) for each dimension in addition to the kernel sum. Cannot be `TRUE` if a permutation operator other than `"none"` is selected.
`compute.ocg` | a logical specifying whether or not to return a separate result for each unordered and ordered dimension, where the product kernel term for that dimension is evaluated at an appropriate reference category. This is used primarily in `np` to compute ordered and unordered categorical gradients. See details.
`return.kernel.weights` | a logical specifying whether or not to return the matrix of generalized product kernel weights. Defaults to `FALSE`. See details.

### Details

`npksum` exists so that you can create your own kernel objects with or without a variable to be weighted (default Y=1). With the options available, you could create new nonparametric tests or even new kernel estimators. The convolution kernel option would allow you to create, say, the least squares cross-validation function for kernel density estimation.

`npksum` uses highly-optimized C code that strives to minimize its ‘memory footprint’, while there is low overhead involved when using repeated calls to this function (see, by way of illustration, the example below that conducts leave-one-out cross-validation for a local constant regression estimator via calls to the `R` function `nlm`, and compares this to the `npregbw` function).

`npksum` implements a variety of methods for computing multivariate kernel sums (p-variate) defined over a set of possibly continuous and/or discrete (unordered, ordered) data. The approach is based on Li and Racine (2003) who employ ‘generalized product kernels’ that admit a mix of continuous and discrete data types.

Three classes of kernel estimators for the continuous data types are available: fixed, adaptive nearest-neighbor, and generalized nearest-neighbor. Adaptive nearest-neighbor bandwidths change with each sample realization in the set, x[i], when estimating the kernel sum at the point x. Generalized nearest-neighbor bandwidths change with the point at which the sum is computed, x. Fixed bandwidths are constant over the support of x.

`npksum` computes sum(t(W[j]) %*% Y[j] * K(X[j])), where W[j] represents a row vector extracted from W. That is, it computes the kernel weighted sum of the outer product of the rows of W and Y. In the examples, the uses of such sums are illustrated.

`npksum` may be invoked either with a formula-like symbolic description of variables on which the sum is to be performed or through a simpler interface whereby data is passed directly to the function via the `txdat` and `tydat` parameters. Use of these two interfaces is mutually exclusive.

Data contained in the data frame `txdat` (and also `exdat`) may be a mix of continuous (default), unordered discrete (to be specified in the data frame `txdat` using the `factor` command), and ordered discrete (to be specified in the data frame `txdat` using the `ordered` command). Data can be entered in an arbitrary order and data types will be detected automatically by the routine (see `np` for details).

Data for which bandwidths are to be estimated may be specified symbolically. A typical description has the form `dependent data ~ explanatory data`, where `dependent data` and `explanatory data` are both series of variables specified by name, separated by the separation character '+'. For example, `y1 ~ x1 + x2` specifies that `y1` is to be kernel-weighted by `x1` and `x2` throughout the sum. See below for further examples.

A variety of kernels may be specified by the user. Kernels implemented for continuous data types include the second, fourth, sixth, and eighth order Gaussian and Epanechnikov kernels, and the uniform kernel. Unordered discrete data types use a variation on Aitchison and Aitken's (1976) kernel, while ordered data types use a variation of the Wang and van Ryzin (1981) kernel (see `np` for details).

The option `operator=` can be used to ‘mix and match’ operator strings to create a ‘hybrid’ kernel provided they match the dimension of the data. For example, for a two-dimensional data frame of numeric datatypes, `operator=c("normal","derivative")` will use the normal (i.e. PDF) kernel for variable one and the derivative of the PDF kernel for variable two. Please note that applying operators will scale the results by factors of h or 1/h where appropriate.

The option `permutation.operator=` can be used to ‘mix and match’ operator strings to create a ‘hybrid’ kernel, in addition to the kernel sum with no operators applied, one for each continuous dimension in the data. For example, for a two-dimensional data frame of numeric datatypes, `permutation.operator=c("derivative")` will return the usual kernel sum as if `operator = c("normal","normal")` in the `ksum` member, and in the `p.ksum` member, it will return kernel sums for `operator = c("derivative","normal")`, and `operator = c("normal","derivative")`. This makes the computation of gradients much easier.

The option `compute.score=` can be used to compute the gradients with respect to h in addition to the normal kernel sum. Like permutations, the additional results are returned in the `p.ksum`. This option does not work in conjunction with `permutation.operator`.

The option `compute.ocg=` works much like `permutation.operator`, but for discrete variables. The kernel is evaluated at a reference category in each dimension: for ordered data, the next lowest category is selected, except in the case of the lowest category, where the second lowest category is selected; for unordered data, the first category is selected. These additional data are returned in the `p.ksum` member. This option can be set simultaneously with `permutation.operator`.

The option `return.kernel.weights=TRUE` returns a matrix of dimension ‘number of training observations’ by ‘number of evaluation observations’ and contains only the generalized product kernel weights ignoring all other objects and options that may be provided to `npksum` (e.g. `bandwidth.divide=TRUE` will be ignored, etc.). Summing the columns of the weight matrix and dividing by ‘number of training observations’ times the product of the bandwidths (i.e. `colMeans(foo$kw)/prod(h)`) would produce the kernel estimator of a (multivariate) density (`operator="normal"`) or multivariate cumulative distribution (`operator="integral"`).

### Value

`npksum` returns a `npkernelsum` object with the following components:

`eval` | the evaluation points
`ksum` | the sum at the evaluation points
`kw` | the kernel weights (when `return.kernel.weights=TRUE` is specified)

### Usage Issues

If you are using data of mixed types, then it is advisable to use the `data.frame` function to construct your input data and not `cbind`, since `cbind` will typically not work as intended on mixed data types and will coerce the data to the same type.

### Author(s)

Tristen Hayfield tristen.hayfield@gmail.com, Jeffrey S. Racine racinej@mcmaster.ca

### References

Aitchison, J. and C.G.G. Aitken (1976), “ Multivariate binary discrimination by the kernel method,” Biometrika, 63, 413-420.

Li, Q. and J.S. Racine (2007), Nonparametric Econometrics: Theory and Practice, Princeton University Press.

Li, Q. and J.S. Racine (2003), “Nonparametric estimation of distributions with categorical and continuous data,” Journal of Multivariate Analysis, 86, 266-292.

Pagan, A. and A. Ullah (1999), Nonparametric Econometrics, Cambridge University Press.

Scott, D.W. (1992), Multivariate Density Estimation. Theory, Practice and Visualization, New York: Wiley.

Silverman, B.W. (1986), Density Estimation, London: Chapman and Hall.

Wang, M.C. and J. van Ryzin (1981), “A class of smooth estimators for discrete distributions,” Biometrika, 68, 301-309.

### Examples

    ## Not run:
    # EXAMPLE 1: For this example, we generate 100,000 observations from a
    # N(0, 1) distribution, then evaluate the kernel density on a grid of 50
    # equally spaced points using the npksum() function, then compare
    # results with the (identical) npudens() function output

    n <- 100000
    x <- rnorm(n)
    x.eval <- seq(-4, 4, length=50)

    # Compute the bandwidth with the normal-reference rule-of-thumb

    bw <- npudensbw(dat=x, bwmethod="normal-reference")

    # Compute the univariate kernel density estimate using the 100,000
    # training data points evaluated on the 50 evaluation data points,
    # i.e., 1/nh times the sum of the kernel function evaluated at each of
    # the 50 points

    den.ksum <- npksum(txdat=x, exdat=x.eval, bws=bw$bw)$ksum/(n*bw$bw[1])

    # Note that, alternatively (easier perhaps), you could use the
    # bandwidth.divide=TRUE argument and drop the *bw$bw[1] term in the
    # denominator, as in
    # npksum(txdat=x, exdat=x.eval, bws=bw$bw, bandwidth.divide=TRUE)$ksum/n

    # Compute the density directly with the npudens() function...

    den <- fitted(npudens(tdat=x, edat=x.eval, bws=bw$bw))

    # Plot the true DGP, the npksum()/(nh) estimate and (identical)
    # npudens() estimate

    plot(x.eval, dnorm(x.eval), xlab="X", ylab="Density", type="l")
    points(x.eval, den.ksum, col="blue")
    points(x.eval, den, col="red", cex=0.2)
    legend(1, .4,
           c("DGP", "npksum()",
           "npudens()"),
           col=c("black", "blue", "red"),
           lty=c(1, 1, 1))

    # Sleep for 5 seconds so that we can examine the output...

    Sys.sleep(5)

    # EXAMPLE 2: For this example, we first obtain residuals from a
    # parametric regression model, then compute a vector of leave-one-out
    # kernel weighted sums of squared residuals where the kernel function is
    # raised to the power 2. Note that this returns a vector of kernel
    # weighted sums, one for each element of the error vector. Note also
    # that you can specify the bandwidth type, kernel function, kernel order
    # etc.

    data("cps71")
    attach(cps71)

    error <- residuals(lm(logwage~age+I(age^2)))

    bw <- npreg(error~age)

    ksum <- npksum(txdat=age,
                   tydat=error^2,
                   bws=bw$bw,
                   leave.one.out=TRUE,
                   kernel.pow=2)

    ksum

    # Obviously, if we wanted the sum of these weighted kernel sums then,
    # trivially,

    sum(ksum$ksum)

    detach(cps71)

    # Sleep for 5 seconds so that we can examine the output...

    Sys.sleep(5)

    # Note that weighted leave-one-out sums of squared residuals are used to
    # construct consistent model specification tests. In fact, the
    # npcmstest() routine in this package is constructed purely from calls
    # to npksum(). You can type npcmstest to see the npcmstest()
    # code and also examine some examples of the many uses of
    # npksum().

    # EXAMPLE 3: For this example, we conduct local-constant (i.e.,
    # Nadaraya-Watson) kernel regression. We shall use cross-validated
    # bandwidths using npregbw() by way of example. Note we extract
    # the kernel sum from npksum() via the `$ksum' argument in both
    # the numerator and denominator.

    data("cps71")
    attach(cps71)

    bw <- npregbw(xdat=age, ydat=logwage)

    fit.lc <- npksum(txdat=age, tydat=logwage, bws=bw$bw)$ksum/
              npksum(txdat=age, bws=bw$bw)$ksum

    plot(age, logwage, xlab="Age", ylab="log(wage)")
    lines(age, fit.lc)

    # Sleep for 5 seconds so that we can examine the output...

    Sys.sleep(5)

    # If you wished to compute the kernel sum for a set of evaluation points,
    # you first generate the set of points then feed this to npksum,
    # e.g., for the set (20, 30, ..., 60) we would use

    age.seq <- seq(20, 60, 10)

    fit.lc <- npksum(txdat=age, exdat=age.seq, tydat=logwage, bws=bw$bw)$ksum/
              npksum(txdat=age, exdat=age.seq, bws=bw$bw)$ksum

    # Note that now fit.lc contains the 5 values of the local constant
    # estimator corresponding to age.seq...

    fit.lc

    detach(cps71)

    # EXAMPLE 4: For this example, we conduct least-squares cross-validation
    # for the local-constant regression estimator. We first write an R
    # function `ss' that computes the leave-one-out sum of squares using the
    # npksum() function, and then feed this function, along with
    # random starting values for the bandwidth vector, to the nlm() routine
    # in R (nlm = Non-Linear Minimization). Finally, we compare results with
    # the function npregbw() that is written solely in C and calls
    # a tightly coupled C-level search routine.  Note that one could make
    # repeated calls to nlm() using different starting values for h (highly
    # recommended in general).

    # Increase the number of digits printed out by default in R and avoid
    # using scientific notation for this example (we wish to compare
    # objective function minima)

    options(scipen=100, digits=12)

    # Generate 100 observations from a simple DGP where one explanatory
    # variable is irrelevant.

    n <- 100

    x1 <- runif(n)
    x2 <- rnorm(n)
    x3 <- runif(n)

    txdat <- data.frame(x1, x2, x3)

    # Note - x3 is irrelevant

    tydat <- x1 + sin(x2) + rnorm(n)

    # Write an R function that returns the average leave-one-out sum of
    # squared residuals for the local constant estimator based upon
    # npksum(). This function accepts one argument and presumes that
    # txdat and tydat have been defined already.

    ss <- function(h) {

    # Test for valid (non-negative) bandwidths - return infinite penalty
    # when this occurs

      if(min(h)<=0) {

        return(.Machine$double.xmax)

      } else {

        mean <-  npksum(txdat,
                        tydat,
                        leave.one.out=TRUE,
                        bandwidth.divide=TRUE,
                        bws=h)$ksum/
                 npksum(txdat,
                        leave.one.out=TRUE,
                        bandwidth.divide=TRUE,
                        bws=h)$ksum

        return(sum((tydat-mean)^2)/length(tydat))

      }

    }

    # Now pass this function to R's nlm() routine along with random starting
    # values and place results in `nlm.return'.

    nlm.return <- nlm(ss, runif(length(txdat)))

    bw <- npregbw(xdat=txdat, ydat=tydat)

    # Bandwidths from nlm()

    nlm.return$estimate

    # Bandwidths from npregbw()

    bw$bw

    # Function value (minimum) from nlm()

    nlm.return$minimum

    # Function value (minimum) from npregbw()

    bw$fval

    # Sleep for 5 seconds so that we can examine the output...

    Sys.sleep(5)

    # EXAMPLE 5: For this example, we use npksum() to plot the kernel
    # function itself. Our `training data' is the singleton, 0, and our
    # evaluation data a grid in [-4,4], while we use a bandwidth of 1. By
    # way of example we plot a sixth order Gaussian kernel (note that this
    # kernel function can assume negative values)

    x <- 0
    x.eval <- seq(-4,4,length=500)

    kernel <- npksum(txdat=x,exdat=x.eval,bws=1,
                     bandwidth.divide=TRUE,
                     ckertype="gaussian",
                     ckerorder=6)$ksum

    plot(x.eval,kernel,type="l",xlab="X",ylab="Kernel Function")
    abline(0,0)

    ## End(Not run)

[Package np version 0.60-9 Index]