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The Campbell Collaboration www.campbellcollaboration.org

Computing Effect Sizes for Clustered Randomized Trials

Terri Pigott, C2 Methods Editor & Co-Chair Professor, Loyola University Chicago

[email protected]

Campbell Collaboration Colloquium – August 2011 www.campbellcollaboration.org

Computing effect sizes in clustered trials •  In an experimental study, we are interested in the difference

in performance between the treatment and control group •  In this case, we use the standardized mean difference, given

by

T C

p

Y Yds−

= g g

Treatment group mean

Control group mean

Pooled sample standard deviation

2


Variance of the standardized mean difference

22 ( )

2( )

T C

T C T C

N N dS dN N N N+

= ++

where NT is the sample size for the treatment group, and NC is the sample size for the control group


TREATMENT GROUP CONTROL GROUP

1 2, ,..., TT T T

NY Y Y

TYgCYg

Overall Trt Mean Overall Cntl Mean

1 2, ,..., CC C C

NY Y Y

2TrtS 2

CntlS2pooledS

3


In cluster randomized trials, SMD more complex •  In cluster randomized trials, we have clusters such as

schools or clinics randomized to treatment and control •  We have at least two means: mean performance for each

cluster, and the overall group mean •  We also have several components of variance – the within-

cluster variance, the variance between cluster means, and the total variance

•  Next slide is an illustration



{ } { }11 1 1,... ,...,T T

T T T Tn m m n

Y Y Y YgggTrt Cluster 1 Trt Cluster mT

{ } { }11 1 1,... ,...,C C

C C C Cn m m n

Y Y Y YgggCntl Cluster 1 Cntl Cluster mC

1TY g T

TmY

g 1CY g C

CmY

g

Trt Cluster 1 Mean Trt Cluster mT Mean Cntl Cluster 1 Mean Cntl Cluster mC Mean

TYgg

••• •••

CYggOverall Trt Mean

Overall Cntl Mean

Assume equal sample size, n, within clusters

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The problem in cluster randomized trials •  Which mean is the appropriate mean to represent the

differences between the treatment and control groups? –  The cluster means? –  The overall treatment and control group means averaged

across clusters? •  Which variance is appropriate to represent the differences

between the treatment and control groups? –  The within-cluster variance? –  The variance among cluster means? –  The total variance?


Three different components of variance •  The next slides outline the three components of variance that

exist in a cluster randomized trial •  These components of variance correspond to the variances

that we would estimate when analyzing a cluster randomized trial using hierarchical linear models

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Pooled within-cluster sample variance •  One component of variance is within-cluster variation •  This is computed as the pooled differences between each

observation and its cluster mean

2 2

1 1 1 12

( ) ( )T Cm n m n

T T C Cij i ij i

i j i jW

Y Y Y YS

N M= = = =

− + −

=−

∑∑ ∑∑g g

M is number of clusters: M = mT + mC

N is total sample size: N = n mT + n mC = NT + NC with equal cluster sample sizes, n



{ } { }11 1 1,... ,...,T T

T T T Tn m m n


{ } { }11 1 1,... ,...,C C

C C C Cn m m n


1TY g T

TmY

g 1CY g C

CmY

g


TYgg

••• •••


Overall Cntl Mean

2WS

Within-cluster variance

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Pooled within-group (treatment and control) sample variance of the cluster means •  Another component of variation is the variance among the

cluster means within the treatment and control groups •  This is computed as the pooled difference between each

cluster mean and its overall group (treatment and control) mean

2 2

2 1 1

( ) ( )

2

T Cm mT T C Ci i

i iB T C

Y Y Y YS

m m= =

− + −=

+ −

∑ ∑g gg g gg



{ } { }11 1 1,... ,...,T T

T T T Tn m m n


{ } { }11 1 1,... ,...,C C

C C C Cn m m n


1TY g T

TmY

g 1CY g C

CmY

g


TYgg

••• •••


Overall Cntl Mean

2BS Variance between cluster

means

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Total pooled within-group (treatment and control) variance •  The total variance is the variation among the individual

observations and their group (treatment and control) overall means

2 2

1 1 1 12

( ) ( )

2

T Cm n m nT T C Cij ij

i j i jT

Y Y Y YS

N= = = =

− + −

=−

∑∑ ∑∑gg gg



{ } { }11 1 1,... ,...,T T

T T T Tn m m n


{ } { }11 1 1,... ,...,C C

C C C Cn m m n


1TY g T

TmY

g 1CY g C

CmY

g


TYgg

••• •••


Overall Cntl Mean

2TS Total pooled within-group variance

(treatment and control)

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Just a little distributional theory •  As usual, we will assume that each of our observations

within the treatment and control group clusters are normally distributed about their cluster means, µT

i and µCi with

common within-cluster variance, σ2W, or

2

2

~ ( , ), 1,..., ; 1,...,

~ ( , ), 1,..., ; 1,...,

T T Tij i W

C C Cij i W

Y N i m j n

Y N i m j n

µ σ

µ σ

= =

= =


Just a little more distributional theory •  We also assume that our clusters are sampled from a

population of clusters (making them random effects) so that the cluster means have a normal sampling distribution with means µT

• and µC• and common variance, σ2

B

2

2

~ ( , ), 1,...,

~ ( , ), 1,...,

T T Ti BC C Ci B

N i mN i m

µ µ σ

µ µ σ

=

=g

g

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The three component s of variance are related:

2 2 2T W Bσ σ σ= +


The intraclass correlation, ρ •  This parameter summarizes the relationship among the three

variance components •  We can think of ρ as the ratio of the between cluster variance

and the total variance •  ρ is given by

2 2

2 2 2B B

B W T

σ σρ

σ σ σ= =

+

10


Why is the intraclass correlation important? •  Later, we will see that we need the intraclass correlation to

obtain the values of our standardized mean difference and its variance

•  Also, if we know the intraclass correlation, we can obtain the value of one of the variances knowing the other two. For example:

2 2

2 2

(1 ) /

(1 )W B

W T

σ ρ σ ρ

σ ρ σ

= −

= −


So far, we have •  Discussed the structure of the data •  Seen how to compute three different sampling variances:

S2W, S2

B, and S2T

•  Discussed the underlying distributions of our clustered data •  Seen how the variance components in the distributions of our

data are related to one another •  Next we will see how to estimate the variance components

using our sampling variances •  (Yes, we are building up to the effect sizes…)

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Estimator of σ2W

•  As we might expect, we estimate σ2W using our estimate of

the within-cluster sampling variance, or,

2 2ˆW WSσ =


Estimator of σ2B

•  Unfortunately, the estimate of σ2B is not as straightforward as

for σ2W

•  This is due to the fact that S2B includes some of the within-

cluster variance. •  The expected value for S2

B is

22 WB n

σσ +

12


Thus, an estimator for σ2B is

22 2ˆ WB B

SSn

σ = −


And, given prior information, an estimate of σ2T is

2 2 21ˆT B WnS Sn

σ−⎛ ⎞= + ⎜ ⎟

⎝ ⎠

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Now, we are ready for computing effect sizes •  There are 3 possibilities for computing an effect size in a

clustered randomized trial •  These correspond to the 3 different components of variance

we have been discussing •  The choice among them depends on the research design

and the information reported in the other studies in our meta-analysis


The most common: δW •  This effect size is comparable to the standardized mean

difference computed from single site designs

T C

WW

µ µδ

σ−

= g g

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We estimate δW using dW

T C

WW

Y YdS−

= gg ggTrt & Cntl means

Pooled within-cluster variance


Variance of dW •  Variance of dW depends on the intraclass correlation, sample

sizes, and dW

{ }21 ( 1)

1 2( )

T CW

W T C

dN N nV dN N N M

ρρ

⎛ ⎞⎛ ⎞+ + −= +⎜ ⎟⎜ ⎟− −⎝ ⎠⎝ ⎠

When SB is 0 (no between-group variation among the cluster means), the variance of dW is equal to the variance for the standardized mean difference in a single-site study

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A second estimator: δT •  This estimator uses the total variation in the denominator •  We may compute this effect size when the other studies in

our meta-analysis are also multi-site studies •  The general form is:

T C

TT

µ µδ

σ−

= g g


There are two ways to compute δT •  Method 1: dT1

–  Used when the study reports S2B , the variance of the cluster

means, and S2W , the pooled within-cluster variance

•  Method 2: dT2 –  Used when we the study reports the intraclass correlation, and

S2T , the total variance

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Estimator dT1 : When we have S2B and S2

W

1

2 2

, withˆ

1ˆ

T C

TT

T B W

Y Yd

nS Sn

σ

σ

−=

−⎛ ⎞= + ⎜ ⎟⎝ ⎠

gg gg


The variance of dT1 (a little messy)

{ }

[ ]

1

2 2 2212 2

(1 ( 1) )

1 ( 1) ( 1) (1 )2 ( 2) 2 ( )

T C

T T C

T

N NV d nN N

n n dn M n N M

ρ

ρ ρ

⎛ ⎞+= + − +⎜ ⎟⎝ ⎠

⎡ ⎤+ − − −+⎢ ⎥

− −⎢ ⎥⎣ ⎦

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Estimator dT2 : When we have S2T and ρ

22( 1)1

2

T C

TT

Y Y ndS N

ρ⎛ ⎞− −= −⎜ ⎟ −⎝ ⎠

gg gg


Variance of dT2 (also messy)

{ }

[ ]

2

2 222

(1 ( 1) )

( 2)(1 ) ( 2 ) 2( 2 ) (1 )2( 2) ( 2) 2( 1)

T C

T T C

T

N NV d nN N

N n N n N ndN N n

ρ

ρ ρ ρ ρρ

⎛ ⎞+= + − +⎜ ⎟⎝ ⎠

⎛ ⎞− − + − + − −⎜ ⎟⎜ ⎟− − − −⎝ ⎠

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A third estimator based on S2B

•  We use this when the treatment effect is defined at the level of the clusters, or when the other studies in our meta-analysis have been analyzed using cluster means as the unit of analysis

•  Though it might not be of general interest, we can use this effect size to obtain other effect sizes of interest

•  There are two estimates of this effect


Estimator dB1 : When we have S2B and S2

W

1

22

, withˆ

ˆ

T C

BB

WB B

Y Yd

SSn

σ

σ

−=

= −

gg gg

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The variance of dB1 (a little messy)

{ }

[ ]

1

2 2212 2 2 2

1 ( 1)

1 ( 1) (1 )2( 2) 2( )

T C

B T C

B

m m nV dm m n

nd

M n N M n

ρρ

ρ ρρ ρ

⎛ ⎞+ + −= +⎜ ⎟⎝ ⎠

⎡ ⎤+ − −+⎢ ⎥

− −⎢ ⎥⎣ ⎦


Estimator dB2 : When we have SB and ρ

21 ( 1)T C

BB

Y Y ndS n

ρρ

− + −= gg gg

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The variance of dB2 (a little messy)

{ }

( )

2

22

1 ( 1)

1 ( 1)2( 2)

T C

B T C

B

m m nV dm m n

n dM n

ρρ

ρ

ρ

⎛ ⎞+ + −= +⎜ ⎟⎝ ⎠

+ −

−


IMPORTANT: Can compute any δ from any other effect size and ρ

1 1

1

TW B

T B W

δρδ δ

ρ ρ

δ δ ρ δ ρ

= =− −

= = −

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And, can compute the variance of the effect size using the same transformations

{ } { } { }

{ } { } { }1 1

(1 )

TW B

T B W

VarVar Var

Var Var Var

δρδ δ

ρ ρ

δ δ ρ δ ρ

= =− −

= = −


Where are we now? •  We have looked at the structure of the data •  We have examined the different components of variation •  We have defined estimators for the different components of

variation •  We have outlined the computation of three different effect

sizes, dW , dT , and dB and their associated variances

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But we all know it is not that simple: Some common problems •  We have unequal cluster sample sizes

–  There are formulas for unequal cluster sample sizes that are much more complex

–  We can assume equal cluster sizes since most studies attempt to use equal cluster sizes in the design

•  We don’t know the intraclass correlation coefficient –  We can estimate it from a number of sources listed in the

References –  Several researchers have provided estimates from other studies

and from large-scale national samples


Let’s look at some examples

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From the analysis section


Another clue that we have cluster means and sds reported

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The ICCs are also reported


But I could not find sample sizes within clusters so looked for another report on the intervention

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So, what do we have in this example? •  We have ρ for SAT Math of 0.72 •  There are 10 matched-pairs of schools, with NT = 976, and

NC = 738 (conservative common n = 73) •  We have the mean of the school (cluster) means and SDs for

the treatment and control group at the level of school

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We have all the information needed to compute dB2

2 22

2

(7.26) (7.48) 54.33, 7.372

82.33 78.77 1 (73 1)0.727.37 73(0.72)

0.48 1.005 0.48

B B

B

S S

d

+= = =

− + −=

= =


To compute Var{dB2 }

{ }

( )

210 10 1 (73 1)(0.72)10*10 (73)(0.72)1 (73 1)0.72 (0.48)2(20 2) (73)(0.72)20 52.84 (52.84)(0.48) 0.201 0.013100 52.56 1892.160.214

BV d + + −⎛ ⎞= ⎜ ⎟⎝ ⎠

+ −+

−

⎛ ⎞⎛ ⎞= + = +⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

=

27


Say we want dW in our meta-analysis

0.720.481 1 0.72

0.48(1.60) 0.77

W Bd d ρρ

= =− −

= =


The variance of dW follows the same transformation

{ } { }

{ }

2

1

0.720.214(1 0.72)

0.214(2.57) 0.550.74

W B

W

Var d Var d

SD d

ρρ

⎛ ⎞= ⎜ ⎟⎜ ⎟−⎝ ⎠

⎛ ⎞= ⎜ ⎟−⎝ ⎠= =

=

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If instead, we want dT

{ } { }

{ }

0.48 0.720.48(0.85) 0.41

(0.214)(0.72) 0.1540.392

T B

T W

T

d d

Var d Var d

SD d

ρ

ρ

= =

= =

=

= =

=


Some observations on this example •  The intraclass correlation is large for this measure so we

have a lot of between school variability •  Thus, it is not surprising that none of the effect sizes we

calculated are significantly different from zero •  In this example, we have the intraclass correlation reported,

but we may not be so lucky

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Another example


We might have individual level info in Table 2

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What information do we have? •  mT = 14 schools in the treatment group, mC = 10 schools in

the control group •  For treatment group, sample size average is nT = 3429/14 =

244.9. For control group, sample size average is nC = 1047/10 = 104.7

•  We have individual level means and standard deviations BUT •  We don’t have the intraclass correlation, or other estimates

of the variance

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To compute effect sizes, we need either 2 estimates of the variance, or 1 estimate and ρ

•  We can only compute the estimate of S2T from our 2

estimates of the individual level sds •  We will need to find an estimate for ρ from another source •  From: Murray & Blitstein (2003). Methods to reduce the

impact of intraclass correlation in group-randomized trials. Evaluation Review, 27: 79-103

•  The upper bound of the ICC for youth cohort studies with outcomes focused on diet is 0.0310


Since we have individual level stats:

2 22

2

(3429 1)(0.52) (1047 1)(0.25) 0.223429 1047 2

3.40 2.16 2(104 1)0.03110.47 208 2

2.64 0.97 2.59

T

T

S

d

− + −= =

+ −

− −= −

−

= =

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The variance of dT

{ }

[ ]

{ }

2

2 22

2

2

1456 1040 (1 (104 1)*0.031)1456*1040

(2496 2)(1 0.031) 104(2496 2*104)0.031 2(2496 2*104)0.031(1 0.031)2.592(2496 2) (2496 2) 2(104 1)0.031

0.007 2.59 (0.0002) 0.00830.091

T

T

V d

SD d

+⎛ ⎞= + − +⎜ ⎟⎝ ⎠

⎛ ⎞− − + − + − −⎜ ⎟⎜ ⎟− − − −⎝ ⎠

= + =

=


If we want dW

{ } { }

{ }

2.59 2.641 1 0.031

0.0083 0.00861 1 0.031

0.093

TW

TW

W

dd

Var dVar d

SD d

ρ

ρ

= = =− −

= = =− −

=

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What about other types of effect sizes, like odds-ratios?

•  We do not yet have the same research for odds ratios •  What we can do is inflate the odds ratios from a clustered

trial using recommendations from the Cochrane Handbook


Recommendations from the Cochrane Handbook •  We first compute the design effect given by

1 + (nave – 1) ρ where nave is the average cluster size, and ρ is the intraclass

correlation coefficient •  We divide the total sample sizes and number of events by

the design effect to obtain the corrected numbers to compute the odds-ratio

•  We adjust the variance by computing the variance of the log-odds ratio using the original sample sizes and dividing by the square root of the design effect

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From the Positive Action program

No smoking Smoked

Intervention 937 4.0% of 976 = 39

Control 682 7.6% of 738 = 56


And the intraclass correlation coefficient

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Adjusted values •  Average cluster size: (738+976)/20 = 85.7 •  Design effect: 1 + (85.7 – 1) 0.05 = 5.235

No smoking Smoked

Intervention 937/5.235=178.99 4.0% of 976 = 39/5.235 = 7.45

Control 682/5.235=130.28 7.6% of 738 = 56/5.235 = 10.7


Variance for the log-odds ratio •  OR = (937*56)/(682*39) = 1.97 •  lnOR = ln(1.97) = 0.68 •  SE2(lnOR) = 1/937 + 1/56 + 1/682 + 1/39 = 0.046 •  SE(lnOR) = 0.214 •  Adjusted SE(lnOR) = 0.214/√5.235 = 0.094

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References Hedges, L. V. (2007). Effect sizes in cluster-randomized

designs. Journal of Educational and Behavioral Statistics, 32, 341-370.

Hedges, L. V. & Hedberg, E. C. (2007). Intraclass correlation values for planning group randomized experiments in education. Educational Evaluation and Policy Analysis, 29, 60-87.

Cochrane Handbook: http://www.cochrane-handbook.org/

The Campbell Collaboration www.campbellcollaboration.org

P.O. Box 7004 St. Olavs plass 0130 Oslo, Norway

E-mail: [email protected]

http://www.campbellcollaboration.org

Download - Computing Effect Sizes for Clustered Randomized Trials€¦ · In cluster randomized trials, SMD more complex • In cluster randomized trials, we have clusters such as schools or

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