circular (version 0.3-1)

lm.circular: Circular-Circular and Circular-Linear Regression

Description

Fits a regression model for a circular dependent and circular independent variable or for a circular dependent and linear independent variables.

Usage

lm.circular(..., type=c("c-c", "c-l"))
lm.circular.cc(y, x, order = 1, level = 0.05)
lm.circular.cl(y, x, init, verbose=FALSE, tol=1e-10)
## S3 method for class 'lm.circular.cl':
print(x, digits = max(3, getOption("digits") - 3), signif.stars= getOption("show.signif.stars"), ...)

Arguments

...

arguments passed to lm.circular.cc or to lm.circular.cl depending on the value of type.

type

if type=="c-c" then lm.circular.cc is called otherwise lm.circular.cl is called.

y

vector of data for the dependent circular variable.

x

vector of data for the independent circular variable if type="c-c" or lm.circular.cc is used otherwise a matrix or a vector containing the independent linear variables.

order

order of trigonometric polynomial to be fit. Order must be an integer value. By default, order=1. Used if type="c-c".

level

level of the test for the significance of higher order trigonometric terms. Used if type="c-c".

init

a vector with initial values of length equal to the columns of x.

verbose

logical: if TRUE messages are printed while the function is running.

tol

the absolute accuracy to be used to achieve convergence of the algorithm.

digits

the number of digits to be printed.

signif.stars

logical; if TRUE, P-values are additionally encoded visually as ``significance stars'' in order to help scanning of long coefficient tables. It defaults to the show.signif.stars slot of

Value

• If type=="c-c" or lm.circular.cc is called directly an object of class lm.circular.cc is returned with the following components:
• call
  match.call().
• rho
  square root of the average of the squares of the estimated conditional concentration parameters of y given x.
• fitted
  fitted values of the model.
• data
  matrix whose columns correspond to x and y.
• residuals
  circular residuals of the model.
• coefficients
  matrix whose entries are the estimated coefficients of the model. The first column corresponds to the coefficients of the model predicting the cosine of y, while the second column contains the estimates for the model predicting the sine of y. The rows of the matrix correspond to the coefficients according to increasing trigonometric order.
• p.values
  p-values testing whether the (order + 1) trigonometric terms are significantly different from zero.
• A.k
  is mean of the cosines of the circular residuals.
• kappa
  assuming the circular residuals come from a von Mises distribution, kappa is the MLE of the concentration parameter.
• If type=="c-l" or lm.circular.cl is called directly an object of class lm.circular.cc is returned with the following components:
• call
  match.call().
• x
  the independent variables.
• y
  the dependent variable.
• mu
  the circular mean of the dependent variable.
• se.mu
  an estimated standard error of the circular mean.
• kappa
  the concentration parameter for the dependent variable.
• se.kappa
  an estimated standard error of the concentration parameter.
• coefficients
  the estimated coefficients.
• cov.coef
  covariance matrix of the estimated coefficients.
• se.coef
  standard errord of the estimated coefficients.
• log.lik
  log-likehood.
• t.values
  values of the t statistics for the coefficients.
• p.values
  p-values of the t statistics. Approximated values using Normal distribution.

Details

If type=="c-c" or lm.circular.cc is called directly a trigonometric polynomial of x is fit against the cosine and sine of y. The order of trigonometric polynomial is specified by order. Fitted values of y are obtained by taking the inverse tangent of the predicted values of sin(y) devided by the predicted values of cos(y). Details of the regression model can be found in Sarma and Jammalamadaka (1993). If type=="c-l" or lm.circular.cl is called directly, this function implements the homoscedastic version of the maximum likelihood regression model proposed by Fisher and Lee (1992). The model assumes that a circular response variable theta has a von Mises distribution with concentration parameter kappa, and mean direction related to a vector of linear predictor variables according to the relationship: mu + 2*atan(beta'*x), where mu and beta are unknown parameters, beta being a vector of regression coefficients. The function uses Green's (1984) iteratively reweighted least squares algorithm to perform the maximum likelihood estimation of kappa, mu, and beta. Standard errors of the estimates of kappa, mu, and beta are estimated via large-sample asymptotic variances using the information matrix. An estimated circular standard error of the estimate of mu is then obtained according to Fisher and Lewis (1983, Example 1).

References

Fisher, N. and Lee, A. (1992). Regression models for an angular response. Biometrics, 48, 665-677.

Fisher, N. and Lewis, T. (1983). Estimating the common mean direction of several circular or spherical distributions with different dispersions. Biometrika, 70, 333-341.

Green, P. (1984). Iteratively reweighted least squares for maximum likelihood estimation, and some robust and resistant alternatives. Journal of the Royal Statistical Society, B, 46, 149-192.

Jammalamadaka, S. Rao and SenGupta, A. (2001). Topics in Circular Statistics, Section 8.3, World Scientific Press, Singapore.

Sarma, Y. and Jammalamadaka, S. (1993). Circular Regression. Statistical Science and Data Analysis, 109-128. Proceeding of the Thrid Pacific Area Statistical Conference. VSP: Utrecht, Netherlands.

Examples

# Generate a data set of dependent circular variables.
x <- circular(runif(50, 0, 2*pi))
y <- atan(0.15*cos(x) + 0.25*sin(x), 0.35*sin(x)) + rvonmises(n=50, mu=0,
kappa=5)

# Fit a circular-circular regression model.
circ.lm <- lm.circular(y, x, order=1)
# Obtain a crude plot a data and fitted regression line.
plot.default(x, y)
circ.lm$fitted[circ.lm$fitted>pi] <- circ.lm$fitted[circ.lm$fitted>pi] - 2*pi 

points.default(x[order(x)], circ.lm$fitted[order(x)], type='l')