10.0Systematic Reviews for Single Subject Designs

Source of Effect Size For SSD

3 Classes Typically Reported

– Standard Mean Difference

– Percentage of Non-Overlapping Data (PND)

– Regression

SESSIONS

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

NUMBER CORRECT

14

12

10

8

6

4

2

0

Baseline

(A)

Intervention

(B)

Simulation DatasetBaseline (A) Treatment (B)

1.76 3.64

1.96 3.65

1.50 4.00

2.01 6.51

3.94 7.20

9.70

11.83

2.01

6.51

12.00

X= 2.12 X= 6.71

sd= .74 sd= 3.52

Sources of Effect Size for SSD

1. Standard Mean Difference

• Subtract the mean of the baseline phase from the mean of the intervention phase

• Divided by standard deviation of baseline

Xtx- Xb d =

sdb

Sources of Effect Size for SSD

Xtx- Xb d =

sdb

6.71-2.12 d = = 6.20

.74

SESSIONS

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

NUMBER CORRECT

14

12

10

8

6

4

2

0

SMDall= 7.23SMD3= 8.25

SESSIONS

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

NUMBER CORRECT

14

12

10

8

6

4

2

0

d = 6.20

Interpretation for SMD

• A 6 standard deviation advantage for the treated participants

• .8 standard deviations=large effect

• The SMD for SSD is not the same metric as the SMD for group comparisons

Sources of Effect Size For SSD

2. Percentage of Non-Overlapping Data (PND)

• Identify the highest baseline point

• Count the number of intervention points that exceed the highest baseline point (non-overlapping).

• Calculate the proportion of non-overlapping to total number of intervention points (can’t use if baseline has a zero point)

SESSIONS

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

NUMBER CORRECT

14

12

10

8

6

4

2

0

PND = 7/10 = 70%

Interpretation for PNDPND scale

– 90%+ = Highly Effective

– 70%-90%= Moderately Effective

– 50%-70%= Minimally Effective

– >50% = Ineffective

Sources of Effect Size SSD

3. Mean Baseline Reduction (MBR)

• Subtract the average of last 3 Tx points from the average of the last 3 baseline points

• Divide by the average of the last 3 baseline points

• Multiply by 100 for percent of baseline reduction (MBR)

Simulation DatasetBaseline (A) Treatment (B)

1.76 3.64

1.96 3.65

1.50 4.00

2.01 6.51

3.94 7.20

9.70

11.83

2.01

6.51

12.00

X= 1.77 X= 6.84

1.77 - 6.84MBR = x 100 = 286.0 1.77

(mean percent reduction)

Interpretation for MBR

• Relative effect: 200% is greater than 100%

• ???? There is no standard for magnitude of effect.