Description Usage Arguments Details Value Methods (by class) Author(s) References See Also Examples
These functions are used to smooth frequency distributions.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17  presmoothing(x, ...)
## Default S3 method:
presmoothing(x, smoothmethod = c("none", "average", "bump",
"loglinear"), jmin, asfreqtab = TRUE, ...)
## S3 method for class 'formula'
presmoothing(x, data, ...)
loglinear(x, scorefun, degrees = list(4, 2, 2), grid, rmimpossible,
asfreqtab = TRUE, models, stepup = !missing(models), compare = FALSE,
choose = FALSE, choosemethod = c("chi", "aic", "bic"), chip,
verbose = FALSE, ...)
freqbump(x, jmin = 1e06, asfreqtab = FALSE, ...)
freqavg(x, jmin = 1, asfreqtab = FALSE, ...)

x 
either an object of class “ 
... 
further arguments passed to other methods. For

smoothmethod 
character string indicating the smoothing method to be
used by 
jmin 
for 
asfreqtab 
logical, with default 
data 
an object of class “ 
scorefun 
matrix of score functions used in loglinear presmoothing,
where each column includes a transformation of the score scale or
interactions between score scales. If missing, 
degrees 
list of integer vectors, each one indicating the maximum
polynomial score transformations to be computed for each variable at a given
order of interactions. Defaults ( 
grid 
matrix with one column per margin in 
rmimpossible 
integer vector indicating columns in 
models 
integer vector indicating which model terms should be grouped
together when fitting multiple nested models. E.g., 
stepup 
logical, with default 
compare 
logical, with default 
choose 
logical, with default 
choosemethod 
string, indicating the method for selecting a
bestfitting model when 
chip 
proportion specifying the typeI error rate for model selection
based on 
verbose 
logical, with default 
Loglinear smoothing is a flexible procedure for reducing irregularities in a
frequency distribution prior to equating, where the degree of each
polynomial term determines the specific moment of the observed distribution
that is preserved in the fitted distribution (see below for examples). The
loglinear
function is a wrapper for glm
, and is used to
simplify the creation of polynomial score functions and the fitting and
comparing of multiple loglinear models.
scorefun
, if supplied, must contain at least one score function of
the scale score values. Specifying a list to degrees
is an
alternative to supplying scorefun
. Each list element in
degrees
should be a vector equal in length to the number of variables
contained in x
; there should also be one such vector for each
possible level of interaction between the variables in x
.
For example, the default degrees = list(4, 2, 2)
is recycled to
produce list(c(4, 4, 4), c(2, 2, 2), c(2, 2, 2))
, resulting in
polynomials to the fourth power for each univariate distribution, to the
second power for each twoway interaction, and to the second power for the
threeway interaction.
Terms can also be specified with grid
, which is a matrix with each
row containing integers specifying the powers for each variable at each
interaction term, including main effects. For example, the main effect to
the first power for the total score in a bivariate distribution would be
c(1, 0)
; the interaction to the second power would be c(2, 2)
.
stepup
is used to run nested models based on subsets of the columns
in scorefun
. Output will correspond to models based on columns 1 and
2, 1 through 3, 1 through 4, to 1 through ncol(scorefun)
. This list
of polynomial terms is then used to create a grid
using
expand.grid
. The grid
can also be supplied directly, in which
case degrees
will be ignored.
compare
returns output as an anova
table, comparing model fit
for all the models run with stepup = TRUE
, or by specifying more than
one model in models
. When choose = TRUE
, the arguments
choosemethod
and chip
are used to automatically select the
bestfitting model based on the anova
table from running
compare
.
The remaining smoothing methods make adjustments to scores with low or zero
frequencies. smoothmethod = "bump"
adds the proportion jmin
to
each score point and then adjusts the probabilities to sum to 1.
smoothmethod = "average"
replaces frequencies falling below the
minimum jmin
with averages of adjacent values.
When smoothmethod = "average"
or smoothmethod =
"bump"
, either a smoothed frequency vector or table is returned. Otherwise,
loglinear
returns the following:

when 

when 

when 

when 

when

default
: Default method for frequency tables.
formula
: Method for “formula
” objects.
Anthony Albano [email protected]
Holland, P. W., and Thayer, D. T. (1987). Notes on the use of loglinear models for fitting discrete probability distributions (PSR Technical Rep. No. 8779; ETS RR8731). Princeton, NJ: ETS.
Holland, P. W., and Thayer, D. T. (2000). Univariate and bivariate loglinear models for discrete test score distributions. Journal of Educational and Behavioral Statistics, 25, 133–183.
Moses, T., and Holland, P. W. (2008). Notes on a general framework for observed score equating (ETS Research Rep. No. RR0859). Princeton, NJ: ETS.
Moses, T., and Holland, P. W. (2009). Selection strategies for univariate loglinear smoothing models and their effect on equating function accuracy. Journal of Educational Measurement, 46, 159–176. ETS.
Wang, T. (2009). Standard errors of equating for the percentile rankbased equipercentile equating with loglinear presmoothing. Journal of Educational and Behavioral Statistics, 34, 7–23.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57  set.seed(2010)
x < round(rnorm(1000, 100, 15))
xscale < 50:150
xtab < freqtab(x, scales = xscale)
# Adjust frequencies
plot(xtab, y = cbind(average = freqavg(xtab),
bump = freqbump(xtab)))
# Smooth x up to 8 degrees and choose best fitting model
# based on aic minimization
xlog1 < loglinear(xtab, degrees = 8,
choose = TRUE, choosemethod = "aic")
plot(xtab, as.data.frame(xlog1)[, 2],
legendtext = "degree = 3")
# Add "teeth" and "gaps" to x
# Smooth with formula interface
teeth < c(.5, rep(c(1, 1, 1, 1, .5), 20))
xttab < as.freqtab(cbind(xscale, c(xtab) * teeth))
xlog2 < presmoothing(~ poly(total, 3, raw = TRUE),
xttab, showWarnings = FALSE)
# Smooth xt using score functions that preserve
# the teeth structure (also 3 moments)
teeth2 < c(1, rep(c(0, 0, 0, 0, 1), 20))
xt.fun < cbind(xscale, xscale^2, xscale^3)
xt.fun < cbind(xt.fun, teeth2, xt.fun * teeth2)
xlog3 < loglinear(xttab, xt.fun, showWarnings = FALSE)
# Plot to compare teeth versus no teeth
op < par(no.readonly = TRUE)
par(mfrow = c(3, 1))
plot(xttab, main = "unsmoothed", ylim = c(0, 30))
plot(xlog2, main = "ignoring teeth", ylim = c(0, 30))
plot(xlog3, main = "preserving teeth", ylim = c(0, 30))
par(op)
# Bivariate example, preserving first 3 moments of total
# and anchor for x and y, and the covariance
# between anchor and total
# see equated scores in Wang (2009), Table 4
xvtab < freqtab(KBneat$x, scales = list(0:36, 0:12))
yvtab < freqtab(KBneat$y, scales = list(0:36, 0:12))
Y < as.data.frame(yvtab)[, 1]
V < as.data.frame(yvtab)[, 2]
scorefun < cbind(Y, Y^2, Y^3, V, V^2, V^3, V*Y)
wang09 < equate(xvtab, yvtab, type = "equip",
method = "chained", smooth = "loglin",
scorefun = scorefun)
wang09$concordance
# Removing impossible scores has essentially no impact
xvlog1 < loglinear(xvtab, scorefun, asfreqtab = FALSE)
xvlog2 < loglinear(xvtab, scorefun, rmimpossible = 1:2)
plot(xvtab, cbind(xvlog1,
xvlog2 = as.data.frame(xvlog2)[, 3]))

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