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One-tailed or Two-tailed Tests
In Statisticshypothesistesting,weneed to judgewhether itis a one-tailed or atwo-tailed test
so thatwecan findthecritical valuesin tables such asStandardNormal zDistribution Tableand
t DistributionTable.And then,by comparingteststatisticvaluewiththecritical valueor
whether statisticvaluefallsin thecritical region,wemakea conclusion either to rejectthenull
hypothesisor to fail to rejectthenull hypothesis.
Howcan wetell whether itisa one-tailed ora two-tailedtest? Itdependson theoriginalclaim
in thequestion.Aone-tailedtestlooksfor an “increase”or “decrease”in theparameter
whereas a two-tailed testlooks for a “change” (could beincreaseor decrease) in the parameter.
Therefore,if weseewords suchas“increased,greater,larger,improved andso on”,or
“decreased,less,smallerandso on”intheoriginal claimof a question (>,<areusedin H1), a
one-tail testisapplied.If wordssuch as“change,thesame,different/differenceand soon” are
used in theclaimof thequestion(≠is used in H1),a two-tailed testisapplied.
In a one-tailed test,thecritical regionhasjustonepart (thegreen areabelow).Itcan bea left-
tailed testor a right-tailed test.Left-tailed test:Thecritical regionisin theextremeleftregion
(tail)under thecurve.Right-tailed test:Thecritical regionisin theextremerightregion(tail)
under thecurve.
rcriticalregion critical region
Left-tailed test Right-tailed test
Sign used in H1: ˂ Sign used in H1: >
In two-tailed test,the critical region hastwo parts(thered areasbelow)which are in the two
extreme gerions (tails)underthecurve.
Two-tailed test
Sign used in H1:≠
Example:
Thetrue valueof onetypeof degreeordiplomacannotbequantitatively measured,butwecan
measureitsrelativeimpacton starting salary.GraduatesfromQuebecuniversitieswith a B.A.or
B.Sc. degreehavea mean annual startingsalary of $28,300.Sixty-fiveQuebecgraduateswith a
civil engineering degreearerandomly selected.Theirstarting salarieshavea meanof $36,300.
If thestandard deviation is$1670,usea 0.01level of significanceto testthe claimthatQuebec
graduateswith a civil engineeringdegreehavea mean startingsalarythatis greaterthanthe
mean forgraduateswith a B.A.orB.Sc.degreefromQuebec.
The underlined partof thequestionaboveistheoriginal claimof thisquestion. Thewords
“greater than”showthatitis a one-tailed (righttail) testandthesignusedin H1 is“>”.Thenull
hypothesisand thealternativehypothesisare:
H0: µ = 28,300
H1: µ > 28,300
Note:In MATH1257 ElementaryStatistics,wealwaysuse“=”sign forthenull hypothesisand
the sign of thealternativehypothesisisbasedon theoriginal claimof thequestion. In PSYC
2126 & 2127,weneed to decideon thesymbol used in alternativehypothesisdepending on the
original claimof thequestion first,and then theoppositesymbol will beused in null hypothesis.
So,the null hypothesisand alternativehypothesisshouldbe:
H0: µ ≤ 28,300
H1: µ > 28,300

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One-tailed-Test-or-Two-tailed-Test(1).pdf

  • 1. One-tailed or Two-tailed Tests In Statisticshypothesistesting,weneed to judgewhether itis a one-tailed or atwo-tailed test so thatwecan findthecritical valuesin tables such asStandardNormal zDistribution Tableand t DistributionTable.And then,by comparingteststatisticvaluewiththecritical valueor whether statisticvaluefallsin thecritical region,wemakea conclusion either to rejectthenull hypothesisor to fail to rejectthenull hypothesis. Howcan wetell whether itisa one-tailed ora two-tailedtest? Itdependson theoriginalclaim in thequestion.Aone-tailedtestlooksfor an “increase”or “decrease”in theparameter whereas a two-tailed testlooks for a “change” (could beincreaseor decrease) in the parameter. Therefore,if weseewords suchas“increased,greater,larger,improved andso on”,or “decreased,less,smallerandso on”intheoriginal claimof a question (>,<areusedin H1), a one-tail testisapplied.If wordssuch as“change,thesame,different/differenceand soon” are used in theclaimof thequestion(≠is used in H1),a two-tailed testisapplied. In a one-tailed test,thecritical regionhasjustonepart (thegreen areabelow).Itcan bea left- tailed testor a right-tailed test.Left-tailed test:Thecritical regionisin theextremeleftregion (tail)under thecurve.Right-tailed test:Thecritical regionisin theextremerightregion(tail) under thecurve. rcriticalregion critical region Left-tailed test Right-tailed test Sign used in H1: ˂ Sign used in H1: >
  • 2. In two-tailed test,the critical region hastwo parts(thered areasbelow)which are in the two extreme gerions (tails)underthecurve. Two-tailed test Sign used in H1:≠ Example: Thetrue valueof onetypeof degreeordiplomacannotbequantitatively measured,butwecan measureitsrelativeimpacton starting salary.GraduatesfromQuebecuniversitieswith a B.A.or B.Sc. degreehavea mean annual startingsalary of $28,300.Sixty-fiveQuebecgraduateswith a civil engineering degreearerandomly selected.Theirstarting salarieshavea meanof $36,300. If thestandard deviation is$1670,usea 0.01level of significanceto testthe claimthatQuebec graduateswith a civil engineeringdegreehavea mean startingsalarythatis greaterthanthe mean forgraduateswith a B.A.orB.Sc.degreefromQuebec. The underlined partof thequestionaboveistheoriginal claimof thisquestion. Thewords “greater than”showthatitis a one-tailed (righttail) testandthesignusedin H1 is“>”.Thenull hypothesisand thealternativehypothesisare: H0: µ = 28,300 H1: µ > 28,300 Note:In MATH1257 ElementaryStatistics,wealwaysuse“=”sign forthenull hypothesisand the sign of thealternativehypothesisisbasedon theoriginal claimof thequestion. In PSYC 2126 & 2127,weneed to decideon thesymbol used in alternativehypothesisdepending on the
  • 3. original claimof thequestion first,and then theoppositesymbol will beused in null hypothesis. So,the null hypothesisand alternativehypothesisshouldbe: H0: µ ≤ 28,300 H1: µ > 28,300