function tTest(x, expectedValue) {
    // The mean of the sample
    const sampleMean = mean(x);

    // The standard deviation of the sample
    const sd = standardDeviation(x);

    // Square root the length of the sample
    const rootN = Math.sqrt(x.length);

    // returning the t value
    return (sampleMean - expectedValue) / (sd / rootN);
}
This is to compute [a one-sample t-test](https://en.wikipedia.org/wiki/Student%27s_t-test#One-sample_t-test), comparing the mean
of a sample to a known value, x.

in this case, we're trying to determine whether the
population mean is equal to the value that we know, which is `x`
here. Usually the results here are used to look up a
[p-value](http://en.wikipedia.org/wiki/P-value), which, for
a certain level of significance, will let you determine that the
null hypothesis can or cannot be rejected.

@param {Array<number>} x sample of one or more numbers
@param {number} expectedValue expected value of the population mean
@returns {number} value
@example
tTest([1, 2, 3, 4, 5, 6], 3.385).toFixed(2); // => '0.16'