Directional (onetailed)
Techniques

We wish to answer the same basic question
that was asked with twotailed testing:
"How likely are we to get this sample if the
null hypothesis is true?"
but we test a directional null hypothesis, as stated
below:
We have the same basic four steps of hypothesis testing:
 State the Hypotheses:
 Null hypothesis: Alcohol consumption does not decrease
birth weight. Their weight will not be less than 18
grams (i.e., it will be equal to or greater than 18
grams). In symbols:
 Alternative Hypothesis: Alcohol will decrease birth
weight. The weight will be less than 18 grams.
In symbols:
 Define the decision method:
 Gather Data:
The two experimenters got these different sets of data:
Experiment 1 
Experiment 2 


Sample Mean = 13 
Sample Mean = 16.5 
 Evaluate Null Hypothesis:
We calculate Z for each experiment, and then look up the
P value for the obtained Z, and make a decision. Here's
what happens for each experiment:
Experiment 1 
Experiment 2 
Sample Mean = 13
Z = (1318)/1 = 5.0
p < .0000
Reject Ho
ViSta Applet 
Sample Mean = 16.5
Z = (16.518)/1 = 1.5
p = .0670
Do Not Reject Ho
ViSta Applet 
ViSta's Report for Univariate Analysis of Experiment
1 Data. 

ViSta's Report for Univariate Analysis of Experiment
2 Data. 

