Question 1181946
Ho: ABM has 20% preference
Ha: ABM does not have 20% preference (2-tailed) and critical value |z|>1.645.
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Ho: ABM has less than or =20% preference
Ha: ABM has > 20% preference (right-tail) with critical value z>1.282
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Ho: ABM has greater than or equal to 20% preference.
Ha: ABM has < 20% preference (left-tail) with critical value z < -1.282
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I learned hypothesis tests in that both have to cover the entire possibility, and that generally equal goes with the null hypothesis.
Also, the direction of the data should be noted. Here, the percentage is 19%, so one would expect the alternative hypothesis to be < 20% OR not equal to 20%.  If the alternative hypothesis is greater than 20%, the data don't show it a priori.
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The equation is z=(p hat-p)/sqrt(0.2*0.8/500)
=-0.01/sqrt(.16/500)
=-0.559