Chapter 11: The t Test for Two Related Samples
Chapter 11: The t Test for Two Related Samples
Related t Formulas
€
t = DSD
€
= DSDn
€
t = (X1 − X 2 )S1
2 + S22
n− 2rS1S2
n
€
t = D∑1n−1
n D2∑ − ( D∑ )2[ ]
Group Assignment of Subjects Matched by IQ
Control Reading ProgramSubject IQ Subject IQ
A 120 E 120B 105 F 105C 110 G 110D 95 H 95
Person
Before Treatment
X1
After Treatment
X2 DA 72 64 -8B 68 60 -8C 60 50 -10D 71 66 -5E 55 56 +1
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D∑ = −30
€
D = D∑n
€
=−305
€
=−6
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t = D−μDSD
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s = SSn−1
€
SD = sn
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df = n−1(Where n = number of difference scores)
µD = ?D scores
Population of difference scores
Obtain sample
Makeinference about µD
Scores on Depression Inventory Before and After Treatment
Person
Before Treatment
X1
After Treatment
X2 DA 72 64 -8B 68 60 -8C 60 50 -10D 71 66 -5E 55 56 +1
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D = −30∑
€
D = D∑n
€
=−305
€
=−6
Person
Before Treatment
X1
After Treatment
X2 D D2
A 72 64 -8 64B 68 60 -8 64C 60 50 -10 100D 71 66 -5 25E 55 56 +1 1
-30 254
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SS = D2 − ( D∑ )2
n∑
€
=(254)− (−30)2
5
€
=254 − 9005
€
=254 −180 = 74
D D (D - D)2
-8 -6 (-2)2
-8 -6 (-2)2
-10 -6 (-4)2
-5 -6 (+1)2
+1 -6 (+7)2
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SS = (D−D∑ )2 = 74
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s = SSn−1
€
= 744
€
= 18.5
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=4.3
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SD = sn
€
=4.35
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= 4.32.236
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=1.92
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t =D−μ DSD
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=(−6)− (0)1.92
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=−3.125
What is the effect of relaxation training on severity of asthma symptoms:
1. Measure the number of doses of medication needed to counter asthma attacks (over 1 week)
Relaxation Training
2. Measure the number of doses of medication needed to counter asthma attacks (over 1 week)
1. Ho: µD = 0 (No change in symptoms)
H1: µD ≠ 0 (There is a change…)
= 0.05
2. tcrit(4) = + 2.776
3.
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tobt = D−μ DSD
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SD = sn
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s = SSn−1
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SS = D2∑ − (∑D)2
n
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tobt = −3.72
4. Reject Ho because tobt of -3.72 < tcrit of -2.776
5. Conclusion: Relaxation training significantly reduced the number of asthma attacks, t(5) = -3.72, p < 0.05.
-2.776 0 +2.776
Reject Ho
Reject Ho
PatientWeek Before
TrainingWeek After
Training D D2
A 9 4 -5 25B 4 1 -3 9C 5 5 0 0D 4 0 -4 16E 5 1 -4 16
€
D∑ = −16
€
D2 = 66∑
€
D = D∑n
€
=−165
€
=−3.2
€
SS = D2∑ − ( D∑ )2
n
€
=66 − (−16)2
5
€
=66 − 51.2
€
=14.8
-2.132
Reject Ho
df = 4
ControlReading Program D D2
Matched pair A 6 15 +9 81Matched pair B 5 15 +10 100Matched pair C 11 17 +6 36Matched pair D 6 13 +7 49
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D∑ = +32
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D2 = 266∑
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D = D∑n
€
=+324
€
=+8
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SS = D2∑ − ( D∑ )2
n
€
=266 − (32)2
4
€
=266 − 256
€
=10
€
=266 −10244
Subject Treatment 1 Treatment 2 DA 10 16 +6B 20 24 +4C 30 35 +5D 40 46 +6E 50 54 +4
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D = +5
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X1 = 30
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X 2 = 35
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SS1 =1000
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SS2 = 964
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SSD = 964
Assumptions for Related-Samples t test
1. Observations within each treatment must be independent
2. The population distribution of difference scores must be normal
3. Note: #2 is not a concern as long as sample size is 30 or less
Person X1 (Before) X2 (After)
A 15 15B 11 13C 10 18D 11 12E 14 16F 10 10G 11 19
SubjectBefore
TreatmentAfter
Treatment1 19 132 35 373 20 144 31 25
Control Reading ProgramSubject IQ Subject IQ
A 120 E 120B 105 F 105C 110 G 110D 95 H 95