Getting your data: Sources and samples

Июль 27, 2022

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Getting your data: Sources and samples

Содержание

2. Sources of psychological data and Data collection methods Data sources Data collection methods Behavior Physiological data
3. Why experiment is not a method of data collection? Because it is a method of study
4. Data collection exercise - 15 mins - In groups of 4 think of a Research Question/
5. Sample What does Sample mean? Sample is a limited set of research objects (units) which we
6. Sample and distribution What is distribution? - a relationship between the values of a random variable
7. Descriptive statistics…
8. Exercise A survey of 20 students was conducted to find out how many books they had
9. Normal distribution Properties of any theoretical normal distribution: 1) The curve never approaches horizontal axis. 2)
10. Normal distribution Skewness = asymmetry Kurtosis = flatness
11. Where is NORMAL distribution?
12. What do we know about STANDARD normal distribution? 1) The curve never approaches horizontal axis 2)
13. Why is it important to know what kind of distribution your variables have? Non-parametric tests Parametric
14. Descriptive statistics… Variance Standard deviation(SD) - is the sum of the squared differences from the M
15. Are you tall?
16. When you know so much about distributions, you can compute a height distribution in your group
17. average Less than average More than average When you know Mean and SD, you can estimate
18. Is this result applicable in other groups? Are you tall in other groups? In HSE? In
19. Standard scores (Z-scores) your individual height mean height in a given sample standard deviation in a
20. Standard normal table Shows you a PROBABILITY that all observed values in your sample are lower
21. What is the probability to find people taller than you in… …Guatemala? Mean = 147.3 cm
23. Sample size and standard error We know M and SD in your group And we know
24. Standard error SD Sample size Guatemala: SE = 6.3/ sqrt(15000) = .05 Our group: SE =?
25. Why do bigger samples provide better estimation? Law of Large Numbers In the end the distribution
26. Sampling strategies Probability strategy True random sampling using a random number table (a computer) to select
27. Exercise: Match the statement with the appropriate term
28. I want to study cultural differences…/ I want to study how culture influence… This is possible
29. Recommended reading: Howitt & Cramer, 2011, p. 232-246 (Samples). Supplementary reading: Bakeman, 2000 (Chapter 7 in
30. Why Standardize ... ? Example 2. Here are the students results (out of 60 points): 20,
32. Скачать презентацию

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Sources of psychological data and Data collection methods
Data sources
Data collection methods
Behavior
Physiological

data
Self-reports
Peer-reports
Activity reports (objective/projective)
Biographical or archival data

Observation
Measurement
Focus-groups
Survey
«Archival data»: databases, papers

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Why experiment is not a method of data collection?
Because it is

a method of study organization

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Data collection exercise - 15 mins -
In groups of 4

think of a Research Question/ Hypothesis
What type of data is the most suitable for your RQ or H?
What data collection method is the most suitable?
WHY?

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Sample
What does Sample mean?
Sample is a limited set of research

objects (units) which we use to make general conclusions about the whole population.
Why do we need samples?

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Sample and distribution
What is distribution?
- a relationship between the values

of a random variable and the frequency (or the probability) with which each of these values can be found in a sample (or a population).

Values of variable

Distribution of values

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Descriptive statistics…

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Exercise
A survey of 20 students was conducted to find out how

many books they had read during the past three months (including books for school). The results from those 20 students are shown below. Find the mean, median, and mode for this data.
2, 4, 5, 1, 3, 2, 5, 6, 1, 2, 4, 3, 6, 10, 12, 10, 2, 8, 6, 7
Answers:
Mean = 4.95.
Median = 4.5
Mode = 2.

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Normal distribution
Properties of any theoretical normal distribution:
1) The curve never approaches

horizontal axis.
2) Symmetrical around the mean.
3) Skewness = 0 and kurtosis = 0.
Standard normal distribution is a special case of theoretical n.d. with 2 properties:
1) μ = 0, σ = 1;
2) area under the curve = 1, and integral of (-∞; z] can be interpreted as probability of finding values equal to or below Z.

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Normal distribution
Skewness = asymmetry
Kurtosis = flatness

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Where is NORMAL distribution?

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What do we know about STANDARD normal distribution?
1) The curve never

approaches horizontal axis
2) Symmetrical around the mean
3) Skewness = 0 and kurtosis = 0
4) Mean = 0, SD = 1
5) Mean= mode= median =0

Example 1. If you get a score of 90 in Math and 95 in English, you might think that you are better in English than in Math. However, in Math, your score is 2 standard deviations above the mean. In English, it’s only one standard deviation above the mean. It tells you that in Math, your score is far higher than most of the students (your score falls into the tail)

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Why is it important to know what kind of distribution your

variables have?

Non-parametric tests

Parametric tests

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Descriptive statistics…
Variance
Standard deviation(SD)
- is the sum of the squared differences

from the M of each score, divided by the total number of scores minus 1
Provides info HOW FAR scores are spread out

Difference from M of ind.score

- square root of variance
It is a quantification of scores variation, and it’s expressed in the same units as the data

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Are you tall?

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When you know so much about distributions, you can compute a

height distribution in your group

your personal height

mean height

sample size

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average
Less than average
More than average
When you know Mean and SD, you

can estimate whether you are tall or not

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Is this result applicable in other groups? Are you tall in other

groups? In HSE? In Russia? To answer this question we should use standard scores

But…

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Standard scores (Z-scores)
your individual height
mean height in a given sample
standard deviation

in a given sample

A very good explanation of Z-scores: https://statistics.laerd.com/statistical-guides/standard-score.php

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Standard normal table
Shows you a PROBABILITY that all observed values in

your sample are lower than Z

The values within the table are the probabilities corresponding to the table type.

The label for rows contains the integer part and the first decimal place of Z.

The label for columns contains the second decimal place of Z.

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What is the probability to find people taller than you in…
…Guatemala?
Mean

= 147.3 cm
SD = 6.3
your Z = (your cm - 147.3)/ 6.3
Then look in Z-table

…Hong Kong?
Mean = 160.1 cm
SD = 5.7
your Z = (your cm - 160.1)/ 5.7
Then look in Z-table

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Sample size and standard error
We know M and SD in your

group
And we know M and SD in Guatemala

Which stats provide more trustworthy description of height in a country?
Why?

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Standard error
SD
Sample size
Guatemala:
SE = 6.3/ sqrt(15000) = .05
Our group:
SE =?
SE depends

on a sample size
The bigger the sample the smaller the SE
The smaller SE the more trustworthy estimations you have

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Why do bigger samples provide better estimation?
Law of Large Numbers
In the

end the distribution of heads vs tails becomes NORMAL (50/50)

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Sampling strategies
Probability strategy
True random sampling
using a random number table (a computer)

to select people from a list, a phone book, etc. (a variety is called ‘systematic random sampling’ = select every nth person);
Stratified sampling / quota sampling
we define the target groups (strata) within our sample (genders, age groups, etc.) and collect respondents from each stratum to get the % you need
Cluster sampling
select the most representative group from a set (a class from a school, a neighborhood from a city
Multi-stage strategies
different strategies used at different sampling stages: e.g., 1) select a school from a city, and 2) select a number of students from that school

Non-probability strategy

Snowball approach:
start with some respondents (e.g., friends), asking each to recruit more people to the study.
Convenience sample:
people at work, students, etc.
Self-selecting sample:
those who agrees to take part in the study; «volunteer bias».

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Exercise: Match the statement with the appropriate term

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I want to study cultural differences…/ I want to study how

culture influence…

This is possible only with representative samples collected in few countries!!!!
A non-representative or a sample from 1 country only cannot help you with this kind of RQ
Open access data:
European Social Survey http://www.europeansocialsurvey.org/
World Values Survey http://www.worldvaluessurvey.org/wvs.jsp
European Values Survey http://www.europeanvaluesstudy.eu/

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Why Standardize ... ?
Example 2. Here are the students results (out

of 60 points):
20, 15, 26, 32, 18, 28, 35, 14, 26, 22, 17
Most students didn't even get 30 out of 60, and most will fail.
The test must have been really hard, so the Prof decides to Standardize all the scores and only fail people 1 standard deviation below the mean.
How many students will fail?
Answer:
The Mean is 23, and the Standard Deviation is 6,6, and these are the Standard Scores:
-0,45, -1,21, 0,45, 1,36, -0,76, 0,76, 1,82, -1,36, 0,45, -0,15, -0,91
Only 2 students will fail (the ones who scored 15 and 14 on the test)

Getting your data: Sources and samples

Содержание

Sources of psychological data and Data collection methodsData sourcesData collection methodsBehaviorPhysiological

Why experiment is not a method of data collection?Because it is

Data collection exercise - 15 mins - In groups of 4

SampleWhat does Sample mean? Sample is a limited set of research

Sample and distributionWhat is distribution? - a relationship between the values

Descriptive statistics…

ExerciseA survey of 20 students was conducted to find out how

Normal distributionProperties of any theoretical normal distribution:1) The curve never approaches

Normal distributionSkewness = asymmetryKurtosis = flatness

Where is NORMAL distribution?

What do we know about STANDARD normal distribution?1) The curve never

Why is it important to know what kind of distribution your

Descriptive statistics…Variance Standard deviation(SD)- is the sum of the squared differences

Are you tall?

When you know so much about distributions, you can compute a

averageLess than averageMore than averageWhen you know Mean and SD, you

Is this result applicable in other groups? Are you tall in other

Standard scores (Z-scores)your individual heightmean height in a given samplestandard deviation

Standard normal tableShows you a PROBABILITY that all observed values in

What is the probability to find people taller than you in……Guatemala?Mean

Sample size and standard errorWe know M and SD in your

Standard errorSDSample sizeGuatemala:SE = 6.3/ sqrt(15000) = .05Our group:SE =?SE depends

Why do bigger samples provide better estimation?Law of Large NumbersIn the

Sampling strategiesProbability strategyTrue random samplingusing a random number table (a computer)