Take home portion of First Year Exam, 2000

Seedling growth and water stress

Researchers conducted an experiment to investigate how water stress in the second summer of growth affects tree seedlings from two species: aspen (as) and jackpines (jp). The experiment region is comprised of 4 blocks; each block consists of 20 seedlings, 10 of which are subjected to the natural environment (W); and 10 of which are shielded from any rain over this period of time (D). Of the 10 trees consisting of a treatment or control group within a block, 5 are aspen (as) and 5 are jackpines (jp). In all, the experiment contains 80 trees. The heights of these trees are recorded weekly from 7 June 1995 to 9 August, giving 8 measurements per tree.

The data are in the file seedlingdata.asc . Hence the data file looks like

trt blk species h1    h2   h3   h4   h5   h6   h7   h8
D   1   jp      13.8  14.6 17.1 17.8 18.3 19.6 20.0 20.1
D   1   jp      12.9  13.9 15.5 16.5 17.2 17.8 18.6 18.9
D   1   jp       9.0   9.0 10.4 11.2 11.5 13.1 13.9 15.5
D   1   jp      16.4  18.2 21.6 23.0 23.3 25.0 26.0 26.6
D   1   jp      19.1  22.2 25.4 26.5 26.9 28.5 29.2 30.1
W   1   jp      18.5  21.3 25.2 27.4 28.3 29.7 30.5 31.1
W   1   jp      11.5  12.6 14.7 16.3 16.6 18.2 18.7 19.4
W   1   jp       7.8  19.8 23.1 24.7 25.7 27.1 27.3 28.6
W   1   jp       9.2  10.8 14.5 16.0 16.8 18.0 19.1 20.7
W   1   jp      11.6  13.4 16.7 17.9 18.6 20.3 21.2 22.8
D   1   as      23.0  21.1 22.8 22.5 22.8 22.9 23.0 23.5
:   :   :         :     :    :    :    :    :    :    :

The columns h1,...,h8 denote the 8 weekly measurements taken in order.

Researchers are interested in the following questions:

Does water stress have an effect on growth rate of seedlings in their second year.
If there is an effect, is it the same for both species? Researchers suspect that the aspen seedlings will suffer more severely under water stress than the jackpines.
How generalizable are the conclusions from this study?

Present a 2 page typed report that addresses the above questions as well as others you may feel are relevant. The presentation may be given in outline form, but it should convey sensible answers to the questions. Try to write the report at a level that a scientists who is familiar with statistics, but not an expert, would understand. You will likely want to give

relevant and useful graphical summaries of the data and/or results,
explanations of any models used,
and your conclusions.

Supplementary documentation may be added to an appendix of no more than 5 pages. However grading will be limited to your 2 page summary. Dave or Merlise will be present Monday evening (until 9:00pm) and Tuesday (from 8:00am until 3:00pm) to answer any questions you might have. The exam is due in Merlise's office by 3:00 on Tuesday May 9. Late answers will not be accepted.