Transportation Statistics and Microsimulation

136    ◾    Transportation Statistics and Microsimulation

shown to be “statistically signicant” if enough observations are taken. It

is up to the analyst to determine whether a given dierence is of practical

importance—even if it is statistically signicant.

e concepts developed in this chapter will be used throughout the

remainder of this textbook and, in all likelihood, will be used through-

out your career. It is imperative that the assumptions and theory are well

understood before moving on to the next chapters.

APPENDIX: WELCH (1938) DEGREES OF FREEDOM

FOR THE UNEQUAL VARIANCE t-TEST

Let

c

S

SS

=

+

1

2

1

2

2

2

,

the Welch degrees of freedom are

df