1.
Development of health
measurement scales – part II
Dr. Rizwan S A, M.D.
If you cannot express in numbers something that you are describing, you probably
have little knowledge about it.
1
4.
Variance = sum of (individual value – mean value)2
----------------------------------------------------------------------------------
no. of values
4
5.
Reliability
• Whether our tool is measuring the attribute in a
reproducible fashion or not
• A way to show the amount of error (random and
systematic) in any measurement
• Sources of error – observers, instruments, instability
of the attribute
• Day to day encounters
– Weighing machine, watch, thermometer
5
6.
Assessing Reliability
• Internal Consistency
– The average correlation among all the items in the tool
• Item-total correlation
• Split half reliability
• Kuder-Richardson 20 & Cronbach‘s alpha
• Multifactor inventories
• Stability
– Reproducibility of a measure on different occasions
• Inter-Observer reliability
• Test-Retest reliability (Intra-Observer reliability)
6
7.
Internal consistency
• All items in a scale tap different aspects of the same
attribute and not different traits
• Items should be moderately corr. with each other and
each item with the total
• Two schools of thought
– If the aim is to describe a trait/behaviour/disorder
– If the aim is to discriminate people with the trait from those
without
• The trend is towards scales that are more internally
consistent
• IC doesn‘t apply to multidimensional scales
7
8.
Item-total correlation
• Oldest, still used
• Correlation of each item with the total score w/o that
item
• For k number of items, we have to calculate k number
of correlations, labourious
• Item should be discarded if r < 0.20
• Best is Pearson‘s R, in case of dichotomous items point-biserial correlation
8
9.
Split half reliability
• Divide the items into two halves and calculate corr.
between them
• Underestimates the true reliability because we are
reducing the length of scale to half (r is directly
related to the no. of items)
– Corrected by Spearman-Brown formula
• Should not be used in
– Highly timed achievement tests
– Chained items
10
10.
KR 20/Cronbach‘s alfa
• KR-20 for dichotomous responses
• Cronbach‘s alfa for more than two responses
• They give the average of all possible split half reliabilities of a
scale
• If removing an item increases the coeff. it should be discarded
• Problems
– Depends on the no. of items
– A scale with two different sub-scales will prob. yield high alfa
– Very high alfa denotes redundancy (asking the same question in
slightly different ways)
– Thus alfa should be more than 0.70 but not more than 0.90
11
11.
• Cronbach‘s basic equation for alpha
n
n
1
1
Vi
Vtest
– n = number of questions
– Vi = variance of scores on each question
– Vtest = total variance of overall scores on the
entire test
13
12.
Calculation of Cronbach‘s coefficient alpha
Example: Assessment of emotional health
During the past month:
Have you been a very nervous person?
Yes No
1
0
Have you felt downhearted and blue?
1
0
Have you felt so down in the dumps that
nothing could cheer you up?
1
0
14
14.
Calculations
Mean score = 2
Sample variance =
CC alpha
1
(2
2)
2
(3
2)
2
(0
(5
1
(. 8 )(. 2 )
1 .5
(. 6 )(. 4 )
(3
2)
2
(2
2)
2
1 .5
k
k
3
2
1)
(% pos ) i (% neg ) i
Var
(. 6 )(. 4 )
2)
1
0 . 86
2
Conclude that this scale has good reliability
16
15.
Multifactor inventories
• More sophisticated techniques
• Item-total procedure – each item should correlate
with the total of its scale and the total of all the scales
• Factor analysis
– Determining the underlying factors
– For eg., if there are five tests
• Vocabulary, fluency, phonetics, reasoning and arithmetic
• We can theorize that the first three would be correlated
under a factor called ‗verbal factor‘ and the last two
under ‗logic factor‘
20
16.
Stability/ Measuring error
• A weighing machine shows weight in the range of
say 40-80 kg and thus an error of 1kg is
meaningful
• A ratio will be more useful,
measurement error / total variability between subjects
But in reality we calculate the ratio
variability between subjects / total variability
(Total variability includes subjects and measurement error)
• So that a ratio of
– 1 indicates no measurement error/perfect reliability
– 0 indicates otherwise
21
17.
• Reliability =
subj. variability / (subj. variability + measurement error)
• Statistically ‗variance‘ is the measure of variability so,
• Reliability =
SD2 of subjects / (SD2 of subjects + SD2 of error)
• Thus reliability is the proportion of the total variance that
is due to the ‗true‘ differences between the subjects
• Reliability has meaning only when applied to specific
populations
22
18.
1
• Measurement error/
total variability between subjects
2
• Variability between subjects/
total variability
3
• Subj. variability/
(subj. variability + measurement error)
4
• SD2 of subjects/
(SD2 of subjects + SD2 of error)
23
19.
Calculation of reliability
• The statistical technique used is ANOVA and
since we have repeated measurements in
reliability, the method is
– repeated measures ANOVA
24
23.
• Classical definition of reliability
• Interpretation is that 88% of the variance is
due to the true variance among patients (aka
Intraclass Correlation Coefficient, ICC)
28
24.
Fixed/random factor
• What happened to the variance due to observers?
• Are these the same observers going to be used or they
are a random sample?
• Other situations where observations may be treated as
fixed is subjects answering ‗same items on a scale‘
29
25.
Other types of reliability
• We have only examined the effect of different
observers on the same behaviour
• But there can be error due to ‗day to day‘ differences,
if we measure the same behaviour a week or two
apart we can calculate ‗intra-observer reliability
coefficient‘
• If there are no observers (self-rated tests) we can still
calculate ‗test-retest reliability‘
31
26.
• Usually high inter-observer is sufficient, but if it is
low then we may have to calculate intra-observer
reliability to determine the source of unreliability
• Mostly measures of internal consistency are reported
as ‗reliability‘, because there are easily computed in a
single sitting
– Hence caution is required as they may not measure
variability due to day to day differences
32
27.
Diff. forms of reliability coefficient
• So far we have seen forms of ICC
• Others
– Pearson product-moment correlation
– Cohen‘s kappa
– Bland – altman analysis
33
28.
Pearson‘s correlation
• Based on regression – the extent to which the relation
between two variables can be described by straight
line
34
29.
Limitations of Pearson‘s R
• A perfect fit of 1.0 may be obtained even if the intercept
is non-zero and the slope is not equal to one unlike with
ICC
• So, Pearson‘s R will be higher than truth, but in practice it
is usually equal to ICC as the predominant source of error
is random variation
• If there are multiple observations then multiple pairwise
Rs are required, unlike the single ICC
• For eg. with 10 observers there will be 45 Pearson‘s Rs
whereas only one ICC
35
33.
• Used when responses are dichotomous/categorical
• When the frequency of positive results is very low or high,
kappa will be very high
• Weighted kappa focuses on disagreement, cells are weighted
according to the distance from the diagonal of agreement
• Weighting can be arbitrary or using quadratic weights (based
on square of the amount of discrepancy)
• Quadratic scheme of weighted kappa is equivalent to ICC
• Also, the unweighted kappa is equal to ICC based on ANOVA
39
34.
Bland and Altman method
• A plot of difference between two observations
against the mean of the two observations
40
36.
• The mean diff. is related to observer variance in ICC, and the SD of
differences if related to the error variance in ICC
• Limits of agreement are calculated as mean difference
the error variance)
2 SE (= to
• Agreement is expressed as the ‘limits of agreement’. The
presentation of the 95% limits of agreement is for visual judgement
of how well two methods of measurement agree. The smaller the
range between these two limits the better the agreement is.
• The question of how small is small depends on the clinical context:
would a difference between measurement methods as extreme as
that described by the 95% limits of agreement meaningfully affect
the interpretation of the results
• Limitation - the onus is placed on the reader to juxtapose the
calculated error against some implicit notion of true variability
42
37.
Issues in Interpretation
SE of measurement and reliability
• R is a dimension-less ratio of variances and so it is difficult to
interpret R in terms of an individual score
• SEM = σ sqrt(1-R)
• If we knew the true score of someone, we can estimate the
limits within which 68% or 95% of the times the observed
value would lie
• Eg. A scale with SD 10 and R 0.8. If the true score was 15, we
can say 68% of the time his observed value will fall between
10.5 to 19.5
43
38.
Standards for magnitude of reliability coeff.
• How
much
reliability
is
good?
Kelly (0.94) Stewart (0.85)
• A test for individual judgment should be higher
than that for research in groups
• For Research purposes –
– Mean score and the sample size will reduce the error
– Conclusions are usually made after a series of studies
– Acceptable reliability is dependent on the sample size
in research
45
39.
Reliability and probability of misclassification
• Depends on the property of the instrument and the
decision of cut point
• Relation between reliability and likelihood of
misclassification
– Eg. A sample of 100, one person ranked 25th and another
50th
– If the R is 0, 50% chance that the two will reverse order on
retesting
– If R is 0.5, 37% chance, with R=0.8, 2.2% chance
• Hence R of 0.75 is minimum requirement for a useful
instrument
46
40.
Improving reliability
• Increase the subject variance relative to the error
variance (by legitimate means and otherwise)
• Reducing error variance
– Observer/rater training
– Removing consistently extreme observers
– Designing better scales
• Increasing true variance
– In case of ‗floor‘ or ‗ceiling‘ effect, introduce items that
will bring the performance to the middle of the scale (thus
increasing true variance)
• Eg. Fair-good-very good-excellent (instead of bad-good)
47
41.
• Ways that are not legitimate
– Test the scale in a heterogeneous population
(normal and bedridden arthritics)
– A scale developed in homogeneous population will
have a larger reliability when used in a
heterogeneous population
• correct for attenuation
48
42.
• Simplest way to increase R is to increase the no. of
items
• True variance increases as the square of items
whereas error variance increases only as the no. of
items
• If the length of the test is triples
– Then Rspearman brown = 3R/ 1 + 2R
49
43.
• In reality the equation overestimates the new
reliability
• We can also use this equation to determine the
length of a test for achieving a pre-decided
reliability
• To improve test-retest reliability – shorten the
interval between the tests
• An ideal approach is the examine all the sources
of variation and try to reduce the larger ones
(generalizability theory)
50
45.
Summary for Reliability
• Pearson R is theoretically incorrect but in
practice fairly close
• Bland and Altman method is analogous to
error variance of ICC but doesn‘t relate this to
the range of observations
• kappa and ICC are identical and most
appropriate
52
46.
Generalizability theory
• Backdrop of classical test theory
– All variance in scores can be divided into true and error
variance (overtly simplistic assumption)
– Don‘t exhaust all possible sources of variance
– Doesn‘t account for interaction between sources of error
variance
• G theory
– Cronbach et al 1972
– Essence is the recognition that in any measurement
situation there are multiple sources of error variance (may
be infinite)
53
48.
Validity
• Two steps to determine usefulness of a scale
– Reliability – necessary but not sufficient
– Validity – next step
• Validity – is the test measuring what it is meant to measure?
• Two important issues
– The nature of the what is being measured
• Temperature Vs. quality of life/social support (physical vs. abstract)
– Relation to the purported cause
• Sr. creatinine is a measure of kidney func. because we know it is regulated
by the kidneys
• But whether students who do volunteer work will become better doctors?
• Since our understanding of human behaviour is far from
perfect, such predictions have to validated against actual
performance
61
51.
Differing perspectives
• Previously validity was seen as demonstrating the properties of
the scale
• Current thinking - what inferences can be made about the
people that have given rise to the scores on these scales?
– Thus validation is a process of hypothesis testing (someone who scores
on test A, will do worse in test B, and will differ from people who do
better in test C and D)
– Researchers are only limited by their imagination to devise experiments
to test such hypotheses
• All types of validity are addressing the same issue of the
degree of confidence we can place in the inferences we can
draw from the scales
64
52.
• Face validity
– On the face of it the tool appears to be measuring what it is
supposed to measure
– Subjective judgment by one/more experts, rarely by
empirical means
• Content validity
– Measures whether the tool includes all relevant domains or
not
– Closely related to face validity
– aka. ‗validity by assumption‘ because an expert says so
• Certain situations where these may not be desired - ?
65
53.
Content validity
• Example – cardiology exam;
– Assume it contains all aspects of the circulatory
system
(physiology,
anatomy,
pathology,
pharmacology etc., etc.,)
– If a person scores high on this test, we can say ‗infer‘
that he knows much about the subject (i.e., our
inferences about the person will right across various
situations)
– In contrast, if the exam did not contain anything about
circulation, the inferences we make about a high scorer
may be wrong most of the time and vice versa
66
54.
• Generally, a measure that includes a more representative
sample of the target behaviour will have more content validity
and hence lead to more accurate inferences
• Reliability places an upper limit on validity (the maximum
validity is the square root of reliability coeff.) the higher the
reliability the higher the maximum possible validity
– One exception is that between internal consistency and
validity (better to sacrifice IC to content validity)
– The ultimate aim of scale is inferential which depends more
on content validity than internal consistency
67
55.
Criterion validity
• Correlation of a scale to an accepted ‗gold standard‘
• Two types
– Concurrent (both the new scale and standard scale are given at the
same time)
– Predictive – the GS results will be available some time in the future
(eg. Entrance test for college admission to assess if a person will
graduate or not)
• Why develop a new scale when we already have a criterion scale?
– Diagnostic utility/substitutability
– Predictive utility (no decision can be made on the basis of new
scale)
• Criterion contamination
– If the result of the GS is in part determined in some way by the
results of the new test, it may lead to an artificially high correlation
68
56.
Construct validity
• Height, weight – readily observable
• Psychological - anxiety, pain, intelligence are abstract
variables and can‘t be directly observed
• For eg. Anxiety – we say that a person has anxiety if he has
sweaty palms, tachycardia, pacing back and forth, difficulty in
concentrating etc., (i.e., we have a hypothesize that these
symptoms are the result of anxiety)
• Such proposed underlying factors are called hypothetical
constructs/ constructs (eg. Anxiety, illness behaviour)
• Such constructs arise from larger theories/ clinical
observations
• Most psychological instruments tap some aspect of construct
69
57.
Early
morning
stiffness
3 or more
joints
involved
esp., small
joints
Rheumatoid
arthritis
Elevated
ESR, RA
factor
X rays
changes
70
58.
Establishing construct validity
• IBS is a construct rather than a disease – it is a
diagnosis of exclusion
• A large vocabulary, wide knowledge and
problem solving skills – what is the underlying
construct?
• Many clinical syndromes are constructs rather
than actual entities (schizophrenia, SLE)
71
59.
• Initial scales for IBS – ruling out other organic
diseases and some physical signs and symptoms
– These scales were inadequate because they lead to
many missed and wrong diagnoses
– New scales developed incorporating demographical
features and personality features
• Now how to assess the validity of this new scale
– Based on my theory, high scorers on this scale should
have
• Symptoms which will not clear with conventional therapy
• Lower prevalence of organic bowel disease on autopsy
72
60.
Differences form other types
1. Content and criterion can be established in one or two
studies, but there is no single experiment that can prove a
construct
• Construct validation is an ongoing process, learning more
about the construct, making new predictions and then
testing them
• Each supportive study strengthens the construct but one
well designed negative study can question the entire
construct
2. We are assessing the theory as well as the measure at the
same time
73
61.
IBS example
• We had predicted that IBS patients will not respond to
conventional therapy
• Assume that we gave the test to a sample of patients
with GI symptoms and treated them with conventional
therapy
• If high scoring patients responded in the same
proportion as low scorers then there are 3 possibilities
– Our scale is good but theory wrong
– Our theory is good but scale bad
– Both scale and theory are bad
• We can identify the reason only from further studies
74
62.
• If an experimental design is used to test the
construct, then in addition to the above
possibilities our experiment may be flawed
• Ultimately, construct validity doesn‘t differ
conceptually from other types of validity
– All validity is at its base some form of construct
validity… it is the basic meaning of validity –
(Guion)
75
63.
Establishing construct validity
• Extreme groups
• Convergent and discriminant validity
• Multitrait-multimethod matrix
76
64.
Extreme groups
• Two groups – as decided by clinicians
– One IBS and the other some other GI disease
– Equivocal diagnosis eliminated
• Two problems
– That we are able to separate two extreme groups implies
that we already have a tool which meets our needs
(however we can do bootstrapping)
– This is not sufficient, the real use of a scale is making much
finer discriminations. But such studies can be a first step, if
the scale fails this it will be probably useless in practical
situations
77
65.
• Convergent validity - If there are two measures for
the same construct, then they should correlate with
each other but should not correlate too much.
E.g. Index of anxiety and ANS awareness index
• Divergent validity – the measure should not correlate
with a measure of a different construct, eg. Anxiety
index and intelligence index
78
66.
Multitrait-multimethod matrix
• Two unrelated traits/constructs each measured by two different methods
• Eg. Two traits – anxiety, intelligence; two methods – a rater, exam
Anxiety
Rater
Anxiety
Intelligence
–
–
–
–
Exam
Intelligence
Rater
Rater
0.53
Exam
0.42
0.79
Rater
0.18
0.17
0.58
Exam
0.15
0.23
0.49
Exam
0.88
Purple – reliabilities of the four instruments (sh be highest)
Blue – homotrait heteromethod corr. (convergent validity)
Yellow – heterotrait homomethod corr. (divergent validity)
Red – heterotrait heteromethod corr. (sh be lowest)
• Very powerful method but very difficult to get such a combination
79
67.
Biases in validity assessment
• Restriction in range
• May be in new scale (MAO level)
• May be in criterion (depression score)
• A third variable correlated to both (severity)
• Eg. A high correlation was found between
MAO levels and depression score in
community based study, but on replicating the
study in hospital the correlation was low
80
70.
Measuring change
• Ultimate goal of most treatment studies is to
induce a change in the patient‘s status
• Controversial views against and for scales which
are more sensitive to change in health status
• Goals of measuring change
– To distinguish between those individuals who change
a lot and those who change little
– To identify correlates of change
– To infer treatment effects from group differences
83
71.
• It is easier to demonstrate a consistent change
in all the subjects, rather than different
amounts of change in different subjects
• Why don‘t we measure change directly?
– Ask patients how have they changed since they
were put on the treatment, because people simply
do not remember how they were at the beginning
(validity of such response is debatable)
– Most defensible way to assess change is to
measure it directly at the beginning of the study
and subsequently on one or more occasions
84
72.
Measures of association
• Reliability
• Sensitivity to change
85
74.
Sensitivity to change from
treatment effects
87
75.
Item response theory
• Limitations of G theory
– Subject/population specific
– Difficult to compare a person‘s score on two or more
different tests (convert to z scores, normality assumption,
not always correct)
– Homoscedasticity assumption that errors are the same at
the ends as in the middle range of scores
– Assumption that all items have equal valences
• Classical test theory – difficult to separate the properties of the test
from the attributes of the people taking it – the tool‘s properties
change as the people tested change, the people‘s properties change
as the test cahnges
91
76.
• IRT – claims to rectify these limitations
– Based on two ‗hard assumptions‘
1. Data are unidimensional (tap only one trait)
2. The probability of answering any item in positive
direction is unrelated to the probability of answering
any other item positively for people with the same
amount of the trait (local independence)
– Two postulates
1. Performance of a subject can be predicted by a set of
factors called ‗traits‘ or abilities, latent traits (theta)
2. The relationship between a person‘s performance on
any item and the underlying trait can be described by
an ‘item characteristic curve’
92
77.
• Some important properties of ICC
– They are ‗ogives‘, usually
– Monotonic; the prob. of answering in a positive
direction consistently increases as the score on the
trait increases
– Differ from each other in three dimensions
• Slope
• Location along the trait
• The flattening out at the bottom
– Can be thought of as ‗imperfect‘ Guttman scales
93
78.
Item characteristic curve
• Q. A is a better discriminator than Q. B
• Q. B is harder than Q. A
94
79.
Different models of ICC
• One parameter model (Rasch model)
– Assumes that all items have equal discriminating ability but
different difficulty
95
80.
• Two parameter model
– Assumes that both discriminating ability and
difficulty differ
96
81.
• Three parameter model
– In addition to the two parameter the lower end of the tail
asymptotes at some probability greater than 0
– Takes care of the fact that when people answer questions by
guessing/ items that are correct by chance
97
82.
Deriving the curves
• Taking a large number of subjects (200 for
one-parameter model, 1000 to estimate the 3
parameter model)
• Random sampling Vs. latent trait model
– In random sampling – it is not necessary to know
much about the items but large pool of items
required
– Latent trait model – fewer items are required but
every item should be known in detail
98
83.
Advantages and disadvantages
• Allows test-free measurement; people can be
compared to one another even if they took different
items
• Eg. Wide Range Achievement Test
• People in different levels can be given different items
and yet be placed on the same scale at the end
(adaptive/tailored testing)
• Not widely used because
– Large sample size needed to estimate the parameters
– Assumptions are difficult to meet
99
84.
Future guidelines for developing health
measurements
1. Articles/manuals should give full description of purpose,
population, intended use
2. Rationale for design of the instrument – conceptual
definition if the object of measurement
3. Describe the ways in which questions were selected
4. Revisions if any should be stated along with reliability and
validity
5. Clear instructions for standard administration and scoring
6. Reliability and validity testing should examine both
internal structure and its relation to alternative
measurements of the concept
7. The tool should be testes by users other than the original
authors
100
85.
Critical appraisal – Rcq - 36
•
•
•
•
•
What is the population in this study?
What is the type of scale?
What is the scaling method used?
Have they missed any method for item generation?
Is Cronbach‘s alfa calculated appropriately and is the
scale reliable?
• Is it appropriate to calculate mean (SD) for each domain?
• Have they established construct validity in this study?
Comment on the MTMM matrix used.
• Can this scale be used to measure treatment effects for
RC?
101
86.
Thank you
―Belief is no substitute for arithmetic‖
— Henry Spencer
102
87.
Scaling Response
Continuous
Direct
estimation
VAS
Discrete
Categorical
Comparative
methods
Adjectival scale
Continuous
Econometric
methods
Specific scaling
Likert
103
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