# Over the same period, determine whether there is any difference in the weight change

August 30, 2017

Question
Over the same period, determine whether there is any difference in the weight change trajectory for babies who meet their nutritional goals versus babies who do not meet their nutritional goals (all goals).

1.
Now that we’ve answered the question about whether there were weight changes for all
subjects (and then changes during the two periods), we want to know whether there
were any differences between the two comparison groups (all nutrition goals met versus
not met) for the same periods (overall, birth to 28 days, and 28 days to discharge).
Formulate three null hypotheses to reflect these new questions

2.
Now let’s look more closely at the separate periods.
As it turns out, the weight change trajectories were not significantly different between
the two groups of babies in all periods.
During which period do we see significant differences in weight change trajectories
between the two nutritional groups?

What is the F statistic for the difference in weight change trajectories of the two groups
during the period in which the trajectories were significantly different? Answer
What is the p value? Answer

What is the change in the means of the two groups during this period:

for the babies who met all their nutritional goals? Answer

for the babies who did NOT meet all their nutritional goals? Answer

grams

grams

3.
Report and interpret the findings with respect to the difference in weight changes for
the two groups for the three hypotheses above

Though we’ve determined that there is a significant difference between nutrition
groups in terms of weight change, we notice that the two groups are different in
terms of the length of stay. So, we wonder whether our previous findings might
be altered if we take NICU length of stay (LOS) into account.
Now, we take the same model we built in the above questions, add length of
stay as a covariate.

4. After controlling for length of stay, we see that the difference in the overall weight
change trajectories between the two groups has changed.
What is the F statistic for the overall weight change trajectory? Answer

What is the p value? Answer

Which of the following is a reasonable conclusion regarding our hypothesis that there is
a difference in weight change trajectories of the two nutritional groups after controlling
for length of stay?
We fail to reject the null and conclude that there is no difference in weight change
trajectories of babies who did and who did not meet their nutritional goals after
controlling for length of stay
We reject the null and conclude that there is no difference in weight change
trajectories of babies who did and who did not meet their nutritional goals after
controlling for length of stay
We reject the null and conclude that even after controlling for length of stay, babies
who met their nutritional goals still had significantly different weight change trajectories
than babies who did not meet their nutritional goals

5. We would like some explanation for why the results changed as dramatically as they
did before and after we controlled for length of stay.
First, what is the mean number of days that babies who met all their nutritional goals

What is the mean number of days that babies who did NOT meet all their nutritional
goals stayed in the NICU? Answer

What proportion of the variance is explained by length of stay?
54.3%
84%
.3%
78.3%
1%

6.
Briefly, why might controlling for LOS change the findings?

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