| Hand in MONDAY:
----- Using table with "z"'s--standard normal.--------- Table use--z: Always sketch a normal curve first, mark the area you are looking for! Do these with the Applet: Normal Density Curve on your CD or at http://www.whfreeman.com/bps/, and check with your table answers. (Uncheck the 2-tail box for most uses. Mean 0, s.d. 1) p.80 3.10 z's to proportions, using Table A. - - - - - - - - - - - - - Begin the rest, especially the Applet work, but don't hand in--will be part of Day 11's work --------- "Backward"--z ---------- Always sketch a normal curve first, roughly mark the proportion=area you are given. p.86, 3.13 (backward z) Do with table, check using Applet: Normal Density Curve on your CD or at http://www.whfreeman.com/bps/. p. 89, 3.52 Quartiles of normal dist. Use the Applet and also, use table A to find the quartiles. Your answers may differ in the second decimal place because the Applet only goes by .02's on the z-axis --.64, .66, .68... and Table A goes by .01's. = = = = Using table with "x"'s--"raw" values. = = = = Begin these by drawing and labeling the appropriate normal curve for each question, leaving space for computation. Then use the Applet: Normal Density Curve on your CD or at http://www.whfreeman.com/bps/. to find the required values. Write these on your paper. Next, calculate the values using Table A. Your answers from each method should be very close (the Table gives a bit more accuracy than the Applet.) p. 87, 3.37 Jacob's score, and 3.39 top score. Mean, s.d. are before 3.32 on p. 86. p. 87, 3.46 surprising difference in tails A. , What proportion of pregnancies last 310 days or more? Find Mean and s.d. in p.74, 3.7 (see below **) p. 80-81 3.11 and 3.12 (locomotive adhesion, 2 dist's) -------- "Backward Normal"----------- Begin these by drawing and labeling the appropriate normal curve for each question, leaving space for computation. Then use the Applet: Normal Density Curve on your CD or at http://www.whfreeman.com/bps/. to find the required values. Write these on your paper. Next, calculate the values using Table A. Your answers from each method should be very close (the Table gives a bit more accuracy than the Applet.) p. 83, 3.14 IQ test p. 87, 3.41 Abigail, top 20%. Mean, s.d. are before 3.32 on p. 86. p. 87, 3.42 quartiles Mean, s.d. are before 3.32 on p. 86. p. 179, 7.27 breaking bolts, (a, b +). For (a), think carefully about which side of 90 you want: Does a bolt that breaks at 95 ksi qualify? Does a bolt that breaks at 85 ksi qualify? ALSO: If they test every bolt and throw away all bolts that break at 70 ksi or below, what proportion do they throw away? |
Read, to discuss | Optional (more
practice) p. 86 3.30 z's to proportions "Backward" p. 87, 3.43, quintiles Mean, s.d. are before 3.32 on p. 86. Quintiles are used by the government to report much economic census data. |
Dear Abby: You wrote in your column that a woman is pregnant for 266 days. Who said so? I carried my baby for ten months and five days, and there is no doubt about it because I know the exact date my baby was conceived. My husband is in the Navy and it couldn't have possibly been conceived any other time because I saw him only once for an hour, and I didn't see him again until the day before the baby was born. I don't drink or run around, and there is no way this baby isn't his, so please print a retraction about that 266-day carrying time because otherwise I am in a lot of trouble.Abby's answer was consoling and gracious but not very statistical:
San Diego Reader
Dear Reader: The average gestation period is 266 days. Some babies come early. Others come late. Yours was late.
The question here is not whether the baby was late.
That
fact is already known. At issue is the credibility of the length of the
delay. Ten months and five days is approximately 310 days, which means
that the pregnancy exceeded the norm by 44 days. [How unusual is that?]
Standardizing: A way of comparing an
individual
against
its pack.
Comparing individuals from different packs, each relative to its own.
Removes "units of measurement" from the discussion.
Enables use of the standard normal table.
Examples: ("Classic IQ test", mean 110, s.d. 25)
85 is 1 s.d. below
the mean. Computation: z = (85 –
110)/25
= (–25 raw points)/25
= –1 s.d. from mean.
145
is
how many s.d.'s above the mean?
Computation: z = (145
– 110)/
25=
(35 raw points above mean)/25
=
1
2/5 = 1.4 s.d. above mean
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-
- - - - - - - - - - - - - - - - - - - -
"What proportion"problems: BPS4e pp. 78-80
or P ( 100 < x <
145)
= P ( –.4 < z < 1.4) = P( z < 1.4) – P(z < –.4) = .9192 –
.3446
= .5746
Read
"Proportion
of x's with 100 <x<145" for P(100<x<145)
- - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - -
"Backward problems" "What
raw (x) value has area ___ to the
left/right
of it?"
BPS4e pp. 81-83.
Sketch the curve, labeled with x values and z values, and the
Area, roughly.
Restate
(if
needed) as "What z value has area A to the LEFT of it."
Look
in body of table for the value closest to A.
Go
to edge(s) of table to find what z that goes with.
Convert
the z to an x: z is
the number
of standard deviations above the mean.
Multiply z by the size of 1
standard
deviation. Now you have distance
above
the mean, measured in raw units.
Add the mean.
Now you have the "raw" value x.
(You have "unstandardized")
Example: What x
value
has 10% of the observations above it? This is the same
x as the one for:
What
x value has 90% of the observations
below
(to the left of) it.
The table gives z = 1.28, approximately.
The "Classic IQ test"score x=
mean
+ z (s.d.) = 110
+ 1.28 (25)= 110
+ 32 = 142
Percentiles: a "Classic IQ
test" score of 142
has 90% of the scores at or below it. 142 is the 90th percentile.
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