Table of Contents

• Testing differences
• 1: How to convince your boss
• 2: Looking at the data
• 3: Is the average different?
• 4: How big is big?
• 5: How to combine SDs
• 6: Combining SDs for fun and profit
• t-calc and t-crit
• 7: Getting to t-calc
• 8: A formula for t-calc
• 9: Degrees of freedom
• 10: The bottom line (finally)
• Putting it all together
• 11: Give it a try
• 12: Once more for luck
• 13: The big picture

Give it a try

As the hotshot fishfood innovator who has just shown how effective your new fishfood is, you would really nail down your case if you could show that other brands don't have your edge. How about practicing with a few more t-tests?

Here's the MalWart brand. See if you can remember each of the calculations to blast this one out of the water.

	n	average	diff	SD	comb. SD	tcalc	tcrit (df= )
MalWart Brand	5	192 g		15 g
Generic Brand	5	173 g		20 g

 

degrees of freedom (df)	tcrit (for p-value = 0.05)
1	12.7
2	4.3
3	3.2
4	2.8
5	2.6
6	2.5
7	2.4
8	2.3
9	2.2
10	2.1
20+	2.0

So, are the two brands different, statistically speaking?