Title: Statistics for the Behavioral Sciences (5th ed.) Gravetter
1Statistics for the Behavioral Sciences (5th ed.)
Gravetter Wallnau
University of GuelphPsychology 3320 Dr. K.
HennigWinter 2003 Term
2Chapter in outline
- Individual Differences in Attachment Quality
- Factors that Influence Attachment Security
- Fathers as Attachment Objects
- Attachment and Later Development
3Honours-No
Honours-Yes
4Measures of Variability
Range Interquartile Range Sum of Squares
(Sample) Variance (Sample) Standard Deviation
5(No Transcript)
6Standard deviation and samples
- Goal of inferential stats is to generalize to
populations from samples - Representativeness? But, samples tend to be less
variable (e.g., tall basketball players) - thus a
biased estimate of variance - Need to correct for the bias by making an
adjustment to derive a more accurate estimate of
the population variability - Variance mean squared deviation sum of
squared deviations/number of scores
7Calculating sd and variance 3 steps(M 6.8
females)
X X-M(Step 1) (X-M)2 (Step 2)
3 -3.8 14.4
4 -2.8 7.8
9 2.2 4.8
Step 3 SS ? (X-M)2
8Calculating variance and sd (contd.)
Step 3 SS ? (X-M)2 - Definition formula(sum
of squared deviations) Alternatively SS ?X2 -
(?X)2/n -computational formula Now correct for
the bias with an adjustment, sample variance s2
SS/n - 1 (sample variance) and
9Thus (text, p. 118)Computational formula
X (X)2
1 1
6 36
4 16
3 9
8 64
7 49
6 36
10Degrees of freedom - two points
Sample of n 3 scores8, 3, 4M 5SS 14
- the sample SS population SS, always
- the difference between the sample mean and the
population mean is the sampling error - you need to know the mean of the sample to
compute the SS thus one variable is dependent on
the rest - df of a sample is n-1 (i.e., the
adjustment) - df (defn) - the number of independent scores.
11Note
- Note. an average (mean) sum/number
- thus, variance is the average deviation from the
mean - mean squared deviation sum of squared
deviations/ - but to calculate sample variance
12Biased and unbiased statisticsTable 4.1
Sample 1
Sample 2
Sample 3
Sample Mean s2 (n) s2 (n-1)
1 0.0 0.0 0.0
2 1.5 2.25 4.5
3 4.5 20.25 40.5
4 1.5 2.25 4.5
9 9.0 0 0.0
total 36 63 126
Sample 4
Sample 5
Sample 6
13Transformation rules
- Adding a constant to each score will not change
the sd - Multiplying each score by a constant causes the
standard deviation to be multiplied by the same
constant
14Variance and inferential stats(seeing patterns)
- conclusion the greater the variability the more
difficult it is to see a pattern - variance in a sample is classified as error
variance (i.e., static noise) - one suit and lots of bad tailors
15Statistics for the Behavioral Sciences (5th ed.)
Gravetter Wallnau
University of GuelphPsychology 3320 Dr. K.
HennigWinter 2003 Term
16Intro to z-scores
- Mean sd as methods of describing entire
distribution of scores - We shift to describing individual scores within
the distribution - uses the mean and sd (as
location markers) - Hang a left (sign is -) at the mean and go down
two standard deviations (number) - 2nd purpose for z-scores is to standardize an
entire distribution
17z-scores and location in a distribution
- Every X has a z-score location
- In a population
? ----gt?
18The z-score formula
- A distribution of scores has a ? 50 and a
standard deviation of ? 8 - if X 58, then z ___ ?
19X to z-score transformationStandardization
- 80 90 100 110 -2
-1 0 1 2 - shape stays the same
- in a z-score distribution is always 0
- the standard deviation is always 1
- procedure
- Bob got a 70 in Biology and a 60 in Chemistry
- for which should he receive a better grade?
20Looking ahead to inferential statistics
Population ? 400 ? 20
Treatment
Sample of n 1
Treated Sample
- Is treated sample different from the original
population? - Compute z-score of sample e.g., if X is extreme
(z2.5), then there is a difference
21Statistics for the Behavioral Sciences (5th ed.)
Gravetter Wallnau
University of GuelphPsychology 3320 Dr. K.
HennigWinter 2003 Term
22Example
- Jar population of 3 checker,1 red dotted, 3
yellow dotted, 3 tiled marbles - if you know the population you know the
probability of picking a n 1 tiled sample - 3/10 (almost a 30 chance)
- but we dont know the population (reality)
- inferential statistics works backwards
23Population
Sample
24Introduction to probability
- probability of A number of outcomes A/
total number of possible outcomes - p(spade) 13/52 ¼ (or 25)
- p (red Ace) ?
- random sample
- each individual in the population has an equal
chance (no selection bias) - if sample gt 1, then there must be constant
probability for each and every selection - e.g., p(jack) if first draw was not a jack?
- sampling with replacement
25(No Transcript)
26God loves a normal curve
2.28
What is the probability of picking a 6 8 (80)
tall person from the population? or p(Xgt80)
80-68/6 2.0 p(zgt2,0) ?
? 68 74 80 ? 6
13.59
34.13
27Unit normal table (Fig. 6.6)
(A) z (B) (C) (D)
.01 .504 .496 .004
.02 .508 .492 .008
B
C
D
28Finding scores corresponding to specific
proportions or ps
z-score
X
unit normaltable
proportionsor ps
29Binomial distribution
- probability of A (heads) p(A)
- probability of B (tails) p(B)
- p q 1.00
1st toss 2nd toss
0 0 0
0 1 1
1 0 1
1 1 2
p.50.25
0 1 2
-With more tosses -gt normal mean increases
(M3 with 6 tosses)
30The normal approximation to the binomial
distribution
- With increases in n the distribution approaches a
normal curve - Given 10 tosses the expectation is to obtain
around 5 heads unlikely to get values far from 5 - Samples with ngt10 (the criteria)
- Mean ? pn(e.g., p (heads given 2 tosses)
½(2)1 - standard deviation ? ?npq
31Example 6.4a (text)
- A PSYC dept. is ¾ female. If a random sample of
48 students is selected, what is p(14 males)?
(i.e., 12 males) - pn¼(48)12?
- qn3/4(48)36?
- p(X 14) are under curve 13.5-14.5
32Example 6.14a (cond.)
12 14 X values .50 .83 z-scores
33Looking ahead to inferential statistics