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Expert: - 3/25/2009


Question
1.A study of iron deficiency among infants compared samples of infants following different feeding regimens. One group contained breast-fed infants, while the children in other group
were fed a standard baby formula without any iron supplements. Here are summary results
on blood hemoglobin levels at 12 months of age.
Group n Mean s
Breast-fed 23 13.3 1.7
Formula 19 12.4 1.8

a. Is there significant evidence that the mean hemoglobin level is higher among breastfed
babies? State null hypothesis and alternate hypothesis and conduct a t-test.

b. Give a 95% confidence interval for the mean difference in hemoglobin level between
the two populations of infants.

2. What do you mean by a seasonal index? Explain ratio to link relatives method of measuring
seasonal variations.


Answer
I found a papper that explains the difference between two means with differing standard deviations.  It is
//stattrek.com/AP-Statistics-4/Unpaired-Means.aspx?Tutorial=AP

Let the SE = sqrt(Sqr(s1)/n1 + sqr(s2)/n2)).
The t statistic is (avg1-avg2)/SE.

A t table will give you whether or not this is significant.
A 95% confindecne interval can be found there as well.
It could be the overall average { (n1*mean1 + n2*mean2)/(n1+n1) }
plus/minu the 95% t statistic with n1 + n2 - 2 degrees of freedom.

2. A seasonal index is meant by noting any difference in which season it is.  For example, you might consider measurements throughout the year and note and statistical difference in them that implied it varied by season.  I would lump the data into four groups by season if it was available.

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