Download Sampling and Measurement

Sampling, Measurement,
Validity and Reliability
Sampling
• Many full scientific texts have been written about
this subject, but it is also a general phenomena
– we all come to conclusions based on samples
of experience that we have had.
• Why sample –
• More economical and efficient
• May be more accurate
• More able to control for biases due to over- or underrepresentation of some population segment
Sampling Terms to Know
• 1. Sampling – the process of selecting a part of
the population to represent the entire population
• 2. Population – an entire aggregation of cases
which meet a designated set of criteria – all
nurses, all BSN nurses, all nurses in Hamilton
County
• Accessible population – all cases which conform to the
criteria and which are accessible for the study
• Target population – the entire field of cases which conform to
the criteria
Sampling Terms to Know
• 3. Sampling Unit – elements or a set of
elements used for sampling – if you want
an element of BSN students, send
questionnaires to BSN schools – the
school is the sampling unit and each
student is an element (the most basic unit
about which information is collected).
Sample Size and Sample Error
• Sample size – Always use the largest sample possible.
In general, a sample size should be at least ten for every
subdivision of the data. 20-30 is preferable. The
absolute size is more important than the relative size.
• Sampling error – the difference between values obtained
from the sample and the values of the whole population.
• Sampling Bias – This occurs when samples are not
carefully selected, i.e. some parts of the population are
left out – internet samples, volunteers
Steps in Sampling
• Identify the target population
• Identify the part of the population that is
accessible to you
• Ask the sample subjects for cooperation
• Select subjects randomly if possible
• Collect data
• Interpret the results based on the sample
– be realistic and conservative
Types of Samples
• Representative sample – a sample in which the
key characteristics of the elements closely
approximate those of the population
• Probability sample – a sample that uses some
form of random selection in choosing the
elements – the researcher can specify the
probability that each element of the population
would be included
• Non-probability sample – a sample in which the
elements are not chosen by random selection
Types of Samples
• Non-probability sample – the elements are
selected by non-random methods. This type of
sampling is more convenient and economical.
– Convenience sample – this is where the researcher
uses the most readily available persons – also called
accidental samples – such as the first persons who
come into a supermarket or a clinic. This is the
weakest method of sampling
– Snowball sample – persons known to the researcher
are asked to participate then the elements are asked
to give names of others they know with the same
characteristic.
Types of Samples
– Quota sampling – the researcher identifies
different strata of the population and determines
the proportions of elements needed from those
various segments of the population (establishes a
quota and fills the quota as the elements present
themselves)
– Purposive sampling (judgmental sampling) the
researcher’s knowledge about the population is
used to handpick the elements to be included so
that the sample meets “the widest type variety” or
the”typical” element. It is good for testing
instruments or validating tests, but it does risk
bias.
– Sequential sampling – Sample one person at a
time until you prove or disprove a statement
(“Seven out of 10 times ASA works better.”)
Quota Sampling
• Males
• Females
Smokers
lll
IIIII III
Non-Smokers
llllI Illll
IIIII III
Types of Sampling
• Probability Sampling
– Simple random sampling – establish a list from which
the sample will be chosen (a sample frame) and
number all elements consecutively. Use a table of
random numbers or a computer to draw numbers.
This guarantees that the differences in attributes of
the sample and of the population are purely a function
of chance and the the probability of selecting a
deviant sample is low. As the size of the sample
increases, the probability of its deviance from the
attributes of the population decreases.
Types of Sampling
– Stratified random sample – mutually exclusive
segments of the population are established by one or
more specifications (male/female; below 30yrs/3045yrs/46yrs and over; diploma/ADN, BSN) and
elements are picked randomly from each stratification
of the population. (Decisions about which strata the
elements belong to are made before the selection as
opposed to quota sampling where the person is
questioned and then put into a stratum.) This method
increases representativeness
– Proportional – elements in proportion to population
– Disproportional – to compare greatly unequal proportions
Stratified Random Sampling
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Numbered list of male smokers - 45
Numbered list of female smokers - 33
Numbered list of male non-smokers - 47
Numbered list of female non-smokers – 37
________________________________
Smokers
Non-smokers
Male
10/45 random
10/33 random
Female 10/47 random
10/37 random
Types of Sampling
– Cluster sampling – this is a process in which a
successive random sampling of units is drawn (states,
then cities, then districts, then blocks, then
households) moving from the largest unit down to the
basic element. It is also called multi-staged sampling.
The sampling error may be larger with it.
– Systematic sampling – the researcher selects every k
th person from a list or a group. It is not random if
you select every 10th person walking by nor is it
random unless you draw the first number to start the
list.
Random Assignment
• Random assignment of subjects to groups
– This eliminates as much systematic bias as possible.
Each subject has an equal chance to be in any of the
study groups and differences are explained on the
basis of the experimental conditions rather than on
differences in subjects.
• Random assignment of treatments to groups
– Exemplified in double-blind studies carried out in
clinical trials. This is particularly useful when the
researcher has to deal with intact groups such as in
classrooms or on hospital units
Measurement
• Measurement is assigning numbers to objects to
represent quantities of attributes or concepts.
• Measurement procedures are operational
definitions of concepts or attributes – the
concept or attribute should really exist although
it may be an abstraction
• Measurement always deals with abstraction –
you don’t measure a person, but a characteristic
of that person
Measurement
• Numbers are assigned to quantify an attribute
– “whatever exists, exists in some amount
and can be measured” The variability of an
attribute is capable of numerical expression
which signifies how much of the attribute is
present in the element.
• Rules for measuring may have to be
invented. The researcher must specify under
what conditions and according to what
criteria, and in what increments, numerical
values are to be assigned.
• Measurement should have a rational
correspondence to reality
Advantages of Measurement
• What would you work with if you did not have
measurement of height, weight, temperature –
intuition, guesses, personal judgment
• Objectivity – scoring minimizes subjectivity.
Analytical procedures are not subjective
• Communication – numbers constitute a nonambiguous language
Levels of Measurement
• Nominal scale – measurement at its weakest –
numbers or other symbols are used to classify
an element – such as a psychiatric diagnostic
number - 295. You can partition a given class of
elements into a set of mutually exclusive
subclasses – 295.30, 295.20. The only
relationship involved is equivalence (=) The
kinds of statistics that can be used with this type
of measurement are modes and frequency
counts. You can test hypotheses regarding
distribution of cases among categories (X2).
Nominal Level
• Marital Status
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Married
Gender
Male Female
60
100
•
140
Single
200
Total
160
340
• Total
200
300
500
• ____________________________________
• Calculate the percentages.
Levels of Measurement
• Ordinal scale – this measurement shows
relationships among classes such as
higher than , more difficult than, etc. It
involves two relations: equivalence (=) and
greater than (>) The researcher can test
hypotheses using non-parametric statistics
of order and ranking such as the
Spearman Rank Order Correlation or the
Mann Whitney U.
Ordinal Level
•
Considerable
Social Support
Moderate
Little
Social Support Social Support
• Bulimics
13
30
57
• Non• Bulimics
35
55
10
Levels of Measurement
• Interval scale – this is similar to the ordinal
scale, but the distance between any two
numbers is of a known size. All parametric
tests are able to be used – mean,
standard deviation, Pearson correlation, Ttest, F-test, etc. It involves three relations:
equivalence (=), greater than (>), and a
known ratio of any two intervals.
Interval Level
• Abused Women Score Before Score After Difference
•
1
36
42
6
•
2
25
40
15
•
3
39
45
6
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4
40
40
0
•
5
41
44
3
•
6
35
40
5
Levels of Measurement
• Ratio scale – it is like the interval scale,
but it has a true zero point as its origin.
You can use arithmetic with it and all
parametric tests as well as those involving
geometric means. It involves four
relationships: equivalence(=), greater
than(>), the known ratio of any two
intervals, and the known ratio of any two
scale values.
Ratio Level
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Experimental Group
Control Group
• Sample Size
35 lbs
35 lbs
• Mean Weight Loss
26 lbs
19 lbs
• Standard Deviation
2 lbs
4.5 lbs
Reliability and Validity - Criteria
for Assessing Measuring Tools
• Every score is part true and part error
• Sources of errors in scores
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Situational contaminants
Response set bias
Transitory personal factors
Administration variations
Instrument clarity
Response sampling (a person scores 95% and 90% on two
tests which claim to test the same thing)
• Instrument format
Reliability
• This is the major criterion for assessing a
measuring instrument’s quality and adequacy. It
is the consistency with which the instrument
measures the attribute it is supposed to be
measuring.
– The reliability of an instrument is not a property of the
instrument, but rather of the instrument when
administered under certain conditions to a certain
sample. (A death anxiety instrument would not
measure the same when given to teenagers as it
measures for geriatric patients.)
Ways to Check Reliability
• Stability (test-retest reliability) – the same test
is given to a sample of individuals on two
occasions, then the scores are compared by
computing a reliability coefficient. (A reliability
coefficient is a correlation coefficient
between the two scores)
• Internal consistency (homogeneity) – all of
the subparts of the instrument must measure
the same characteristic. Use the split-half
technique –split the test items in half, score
each half, then compare the scores using a
correlation coefficient; or compare each item
(by correlation) with the total score (α)
Ways to Check Reliability
• Equivalence – can be tested in two ways
– 1. Using two or more forms of a test to see if they are
equal
– 2. Inter-rater reliability
• Carefully train observers, develop clearly defined,nonoverlapping categories, and use behaviors that are molecular
rather than molar
• Two or more observers watch the same event simultaneously
and independently record variables according to a plan or
code
• Reliability is computed:
– Reliability = number of agreements
number of agreements + number of
disagreements
Ways to Check Reliability
• Interpretation of reliability coefficients
– If you are interested only in group-level comparisons,
a reliability coefficient of .70 or even .60 is sufficient
(male/female, Dr./nurse, smoker/non-smoker)
– If you are interested in decisions about individuals,
such as who gets into school, then a coefficient of .90
or higher is needed
– If the coefficient were .80, then 80% of the scores’
variability would be true variability and 20% would be
extraneous
Ways to Improve Reliability
• Add more items
• Have a more varied group of subjects –
the more homogeneous the group the
lower the reliability coefficient
Validity
• The degree to which an instrument
measures what it is supposed to be
measuring. Validity is difficult to establish.
An instrument that is not reliable cannot be
valid, but, an instrument can be reliable
and still not be valid. (Example – a patient
satisfaction scale does not measure
“quality of nursing care.”)
Aspects of Validity
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Face Validity
Content Validity
Criterion-related validity
Construct validity
Validity
• Face validity – refers to whether the
instrument looks as though it is measuring
the appropriate attribute. It is based on
judgment. There are no objective criteria
used for assessment of its
appropriateness.
Types of Instrument Validity
• Content validity – looks at the sampling
adequacy of the content area – used especially
for tests that measure knowledge of a specific
content area. It is evaluated by examining the
extent to which the content of the test represents
the total domain of behaviors encompassing the
ability being measured. It is usually measured
by expert opinion. It is based on judgment. The
more experts who agree on the content to be
included, the better – a blueprint could be
developed or a content validity index (CVI) could
be developed –see method in Polit, p. 459
Types of Instrument Validity
• Criterion-related validity – this establishes a
relationship between the instrument and some
other criterion that is accepted as measuring the
same attribute. The scores on both should
correlate highly indicating directly how valid the
instrument is.
– Concurrent validity – the criterion measure is obtained
at the same time the test is given
– Predictive validity – the criterion measure is obtained
some time after the test is given and the test is used
to predict future performance on the criterion measure
Types of Instrument Validity
• Construct validity – asks the question: Is the
abstract concept/construct under investigation
being adequately measured with this instrument
– is there a fit between the conceptual definition
and the operational definition of a variable. One
way to test it is through the known groups
technique – groups expected to differ on the
critical attribute are tested and scores should be
different. If the test is a sample of behaviors
characteristic of the construct. Its items must be
representative of the content of the construct. A
good way to support this assumption is to use
factor analysis.
Types of Instrument Validity
• Statistical Conclusion Validity –
determines whether the conclusions drawn
about the relationships are an accurate
reflection of the real world and/or whether
the differences drawn from statistical
analyses are an accurate reflection of the
real world.
Benefits and Limitations of
Statistical Conclusion Methods
• Benefits
– Enhances interpretability of relationships
– Easy and economical
– Can be used with a large number of extraneous
variables
• Limitations
– Requires knowledge of which variables to control to
enhance the independent variable
– Requires statistical sophistication regarding statistical
power and statistical precision
Interpretation of Validity
• Validity cannot be proved but it can be
supported. The researcher does not
validate the instrument itself, but actually
some application of the instrument
Other Criteria for an Instrument
• Efficiency – the number of items, the time it takes to
complete
• Sensitivity – how small a variation in the attribute can
be detected and measured – use item analysis of
tests
• Objectivity – two researchers should agree about its
measurement
• Comprehensibility – subjects can understand what to
do with it
• Balance – to minimize response sets
• Time allowance – adequate time is available for
completion
• Simplicity