Notes on Lab report 2. I assigned 20 points apiece to the following sections: Introduction, Materials and Methods, Results, Data analysis, and Discussion. For late papers, I calculated the fraction of an 8-hour work day that the paper was late, multiplied that fraction by 10 and subtracted the result from the total points to produce the final grade. Between 8 and 24 hours late was considered one whole day late, as I could not be sure when papers were handed in after the office closed and the building was empty.
 
 

1) Introduction. Most of the problems cropped up here. NOBODY really laid out the problem in terms anything like Grand’s paper, namely how is a prehensile tail useful in solving the problems of traveling, foraging, and feeding in the canopy? To do this well, you would need to: 1) describe what the problems are (discontinuities between tree crowns, bending of terminal branches under the weight of heavier monkeys, inability of small feet or hands to grasp fat branches or trunks); 2) how these problems affect particularly animals of the size and weight of a capuchin monkey (smaller animals have less problems with terminal branch bending, heavier ones have more problem of staying on top of thin branches); 3) hypothesize how the prehensile tail could be used to ‘solve’ these problems with special reference to the variables you will be analyzing in the videos. For this last part, you should write out a series of null hypotheses and alternative hypotheses (‘predictions’) for exactly how you expect the prehensile tail to be used on substrates of different orientations, angles, and thicknesses.
 
 

What is a null hypothesis? It is a statement of what you expect to find if the variables you are analyzing are NOT functionally related to each other. Typically, they have the form of "variable X is independent of variable Y", or "variable X is expected to occur with equal probability regardless of the value of variable Y". The alternative hypothesis is a statement of what you expect to find, based on theoretical considerations. It is not enough just to say that you expect there will be an association between the two – there are many possible results that would make no sense at all. Any expected patterns should be justified by theoretical principles or at least common sense – simply stating that "I believe this is true" is not sufficient. For instance, howler monkeys should have problems of falling off or breaking thin branches more than medium or large ones, so you would predict they would use their tail to grab onto supports more if they are on thin branches than on medium or large ones. Note that the goal of any scientific analysis is NEVER to PROVE a particular hypothesis or theory to be true or false. Real scientists can at best make strong probability statements, but this is not the same as proof.
 
 

The Introduction is NOT the place to anticipate your results in detail. You may tease the reader by giving a general brief statement of the overall findings (e.g., "In fact, tail use proved to be strongly associated with some aspects of the substrates used by the monkeys").
 
 

2) Materials and Methods. As James pointed out 2 lab sessions ago, the M&M section needs to provide enough information so that a reader unfamiliar with the subject could in principle go out and duplicate your results. You may cite the lab handout to justify the methods (for lack of a better reference), but still need to describe the methods, criteria, and any problems you had implementing them. Note that you may use a given set of data, observations, video clips, or whatever ONCE AND ONLY ONCE in a given analysis – looking at a video and recording the data several times does NOT give you a bigger sample size, despite what you may think. ALL conventional statistical tests require that the data points be INDEPENDENT – clearly repeating data from the same video would not produce independent data points, because if you knew the first set of data, you could exactly (or nearly exactly) predict what the second set would turn out to be.
 
 

3) Results. You need to describe the results in words, not just using statistics (which give probability values, but not a sense of what the data look like). You should compare the observed results to the hypotheses you set out in the Introduction. Note (again -- James already told you this) that if a statistical test does not yield a value large enough to reject the null hypothesis, you really have NO reason to discuss the result further. Exceptions may be made on occasion for patterns that are nearly significantly different from that expected in the null hypothesis AND in the direction predicted by the alternative hypothesis. Note that it is ALWAYS incorrect to state that "the data were not significant". Data themselves are never significant or not significant; it is only the PATTERN of the data RELATIVE TO the null hypothesis that can be considered significantly different or not. Also, you MAY NOT just look at the patterns in the data and claim that they exist or are significant – you HAVE to back up any such statements with statistics. The reason is that human brains are INCREDIBLY good pattern-finders; they are SO good, that they will find patterns where there aren’t any (in the sense that the data were generated by a random process). Also, note that an hypothesis of no association between X and Y does NOT mean that X and not-X should occur the SAME amount for each value, level, or category of Y. For istance, tail use was NOT expected to occur half the time for each angle category or thickness of substrate. The overall frequency of tail use is dictated by the animals themselves – it could have been only 5% of all the observations, or 95%.
 
 

4) Data analysis. Generally you all did the Chi-square calculations correctly, given the format of your tables. However, several did not follow instructions in handout to combine data in original data tables so that only one ‘factor’ (orientation, angle, OR thickness) was compared to tail use (yes vs. no). Using more complex tables (e.g., combining substrate size and orientation or angle) would be OK, but in practice, the people who used these did not know how to perform the tests and interpret the results. Extra points were given for use of graphs to illustrate the tables. Usually, statistical tests produce conclusions that are or are not true (the results are, or are not, significant at some stated probability criterion), rather than "somewhat" true (never mind that philosophers debate at length about partial truth values).
 
 

5) Discussion. The Discussion is NOT a place to simply repeat the conclusions or inferences from the results. A brief summary of the general outcomes of the Results is appropriate, to remind the reader. You should then have related the results obtained back to the general principles or questions brought up in the Introduction. Few students did this, primarily because few actually provided such general views in the Introduction. As a lesser fallback, you could compare the patterns of tail use to that found in other primate species (as described in Grand’s paper or other references available in the literature). If the results did not come out as expected, here is the place to discuss why not. Possibilities include that 1) the hypothesis was in fact wrong -- if so, find the error in the assumptions or logic that led to an incorrect prediction; 2) you performed the analysis incorrectly; 3) the data were too few for a significant result to be found (but the pattern of the results does agree with the alternative hypothesis); 4) more than one hypothesis may be at work, with contradictory expected patterns (e.g., some physical factors may make it necessary to use tails more on thin substrates, and other physical factors may require more tail use on thick substrates, thus producing little overall pattern). In any case, describe what you would do to improve upon the study design to obtain a more convincing result (convincing does not mean significant – even ‘negative’ results [accepting the null hypothesis] can be more or less convincing).
 
 

6) General stuff: a) again (James did tell you this), DATA is a plural word: "these data show" NOT "this data shows"; b) the possessive form of "it" is written "its" WITHOUT AN APOSTROPHE – "it’s" is a contracted form of "it is"; c) every English sentence (except in ‘creative’ writing) must possess a subject and verb. Verb forms ending in ‘ing’ are not verbs but gerunds, which usually act as adjectives or adverbs. Thus, "The monkey hanging by its tail." is not a sentence. READ OUT LOUD every sentence you write – if you had done this before handing in the assignment, you could NOT have left in many of the ‘sentences’ that you did. I marked non-sentences with NAS (Not A Sentence).