ds.tTest: Runs a student's t-test
In datashield/dsStatsClient: DataSHIELD client site stattistical functions

Description Usage Arguments Details Value Author(s) Examples

Performs one and two sample t-tests on vectors of data.

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ds.tTest(x = NULL, y = NULL, type = "combine",
  alternative = "two.sided", mu = 0, paired = FALSE, var.equal = FALSE,
  conf.level = 0.95, datasources = NULL)

`x`	a character, the name of a (non-empty) numeric vector of data values or a formula of the form 'a~b' where 'a' is the name of a continuous variable and 'b' that of a factor variable.
`y`	a character, the name of an optional (non-empty) numeric vector of data values.
`type`	a character which tells if the test is ran for the pooled data or not. By default `type` is set to 'combine' and a t.test of the pooled data is carried out. If `type` is set to 'split', a t.test is ran for each study separately.
`alternative`	a character specifying the alternative hypothesis, must be one of "two.sided" (default), "greater" or "less". You can specify just the initial letter.
`mu`	a number indicating the true value of the mean (or difference in means if you are performing a two sample test).
`paired`	a logical indicating whether you want a paired t-test.
`var.equal`	a logical variable indicating whether to treat the two variances as being equal. If TRUE then the pooled variance is used to estimate the variance otherwise the Welch. (or Satterthwaite) approximation to the degrees of freedom is used.
`conf.level`	confidence level of the interval.
`datasources`	a list of opal object(s) obtained after login in to opal servers; these objects hold also the data assign to R, as `dataframe`, from opal datasources.

Summary statistics are obtained from each of the data sets that are located on the distinct computers/servers. And then grand means and variances are calculated. Those are used for performing t-test. The funtion allows for the calculation of t-test between two continuous variables or between a continuous and a factor variable; the latter option requires a formula (see parameter dataframe). If a formula is provided all other but 'conf.level=0.95' are ignored.

a list containing the following elements: statistic the value of the t-statistic. parameter the degrees of freedom for the t-statistic. p.value p.value the p-value for the test. conf.int a confidence interval for the mean appropriate to the specified alternative hypothesis. estimate the estimated mean or difference in means depending on whether it was a one-sample test or a two-sample test. null.value the specified hypothesized value of the mean or mean difference depending on whether it was a one-sample test or a two-sample test. alternative a character string describing the alternative hypothesis method a character string indicating what type of t-test was performed

an object of type 'htest' if both x and y are continuous and a list otherwise.

Isaeva, J.; Gaye, A.

{

  # load that contains the login details
  data(logindata)

  # login and assign all the variables
  opals <- datashield.login(logins=logindata,assign=TRUE)

  # Example 1: Run a t.test of the pooled data for the variables 'LAB_HDL' and 'LAB_TSC' - default
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC')

  # Example 2: Run a test to compare the mean of a continuous variable across the two categories of a categorical variable
  s <- ds.tTest(x='D$PM_BMI_CONTINUOUS~D$GENDER')

  # Example 3: Run a t.test for each study separately for the same variables as above
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', type='split')

  # Example 4: Run a paired t.test of the pooled data
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', paired=TRUE)

  # Example 5: Run a paired t.test for each study separately
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', paired=TRUE, type='split')

  # Example 6: Run a t.test of the pooled data with different alternatives
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', alternative='greater')
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', alternative='less')

  # Example 7: Run a t.test of the pooled data with mu different from zero
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', mu=-4)

  # Example 8: Run a t.test of the pooled data assuming that variances of variables are equal
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', var.equal=TRUE)

  # Example 9: Run a t.test of the pooled data with 90% confidence interval
  ds.tTest(x='D$LAB_HDL', y='D$LAB_TSC', conf.level=0.90)

  # Example 10: Run a one-sample t.test of the pooled data
  ds.tTest(x='D$LAB_HDL')
  # the below example should not work, paired t.test is not possible if the 'y' variable is missing
  # ds.tTest(x='D$LAB_HDL', paired=TRUE)

  # clear the Datashield R sessions and logout
  datashield.logout(opals)

}