Langston, Research Methods, Notes 13 -- Strong inference
 
I.  Goals:
A.  What is strong inference?
B.  Digression to discuss causation.
B.  How does this apply to real-world research?
C.  How does it all tie together?
 
II.  What is strong inference?  (Platt, 1964)  Until now we've pretended that experiments happen in isolation.  In reality, that is almost never the case.  The experimental enterprise is cumulative.  New experiments build on old experiments; research is conducted as part of a larger program of investigation.  What we'll do now is look at ways in which series of experiments can be combined to test theories.
Our first example of this will be strong inference.  At its core, strong inference works like statistical hypothesis testing.  You set up a group of mutually exclusive and exhaustive hypotheses and then attempt to rule out all but one of those hypotheses.  The difference is that we're getting our hypotheses from theories that exist to explain the phenomenon of interest.  We can have as many hypotheses as there are theories.
A.  The steps in strong inference:
1.  Devise multiple hypotheses.  You can do this based on your own intuitions (not so hot) or based on a careful review of the literature (much better).  The goal is to gather together all proposed answers to your question of interest.  Why do this?
a.  It protects us against our bias to try and confirm our first idea.  Instead, we're forced to think about all of the possibilities.
b.  If our “pet” hypothesis isn't supported by the data, we can still make some sort of statement (as opposed to saying “I guess we were wrong” we can say “the data supported this hypothesis”).
This step is the most important.  If you fail to generate all alternatives you won't be guaranteed that your data will be consistent with any of the hypotheses, and you'll have a harder time convincing people that you're correct.  As an example, imagine that I have a hypothesis which you didn't consider that generates the same predictions as the hypothesis that was consistent with your data.  You want to say your hypothesis is correct, I can argue that based on your data mine's just as good, and you're out of luck.
2.  Design an experiment to test between these alternatives.  Ideally, the outcome of one experiment will be consistent with only one of your alternatives and inconsistent with all the rest.  Sometimes, it's not possible to do it all in one experiment.  In this case, you'll carry out a series of experiments.
Note the importance of predictions.  As you design the experiment you need to constantly check to be sure that each hypothesis makes different predictions (like no main effects but an interaction vs. one main effect and no interaction).  If they don't all make different predictions, then you won't be able to rule them all out, and you're wasting your time.
3.  Carry out your experiment.  This step involves most of the first part of this course.  If you run a clean experiment, your conclusions will carry more weight.  If your experiment is full of confounds, your conclusions are worthless.
Sometimes, you repeat these steps several times to get to just one hypothesis.  This might happen if you can't force all of the hypotheses to make different predictions, or if you realize after the experiment that a slightly different version of one of the hypotheses might still be OK, and you want to refine it even more.  It's kind of like tournament bowling:  The two lowest contenders meet.  The winner advances to meet the next highest contender.  The winner of that round advances...
B.  Characteristics of strong inference research:
1.  Consideration of multiple hypotheses.  This helps to save you from your natural bias to always do confirmation research.  It also means that this kind of research is rarely done in an exploratory context (if you don't know anything about a phenomenon, how can you test theories to explain it?).
2.  Organization of experiments.  There's a particular structure to the sequence of experiments and this organization is largely responsible for the power of the technique.
3.  Increment by exclusion.  We gain in knowledge by ruling out alternatives.  It seems paradoxical, but by eliminating possibilities we have a much better estimate of what the correct answer is.
 
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IV.  Digression to discuss causation.  When we talk about A causing B, there are two kinds of possible causal relationships:
A.  Necessary cause:  If A is necessary to cause B, then when A is absent you won't find B.
B.  Sufficient cause:  If A is sufficient to cause B, then when A and A alone is present you'll find B.
C.  These two are independent.  The combinations:
1.  Neither necessary nor sufficient:  sunlight is NNNS for photosynthesis.
2.  Necessary but not sufficient:  light is NBNS for vision.
3.  Sufficient but not necessary:  watering the grass is SBNN for the ground becoming wet.
4.  Necessary and sufficient:  temperatures below 0°C are NAS for water to freeze.
D.  These are related to the logic of hypothesis testing:
1.  The two valid forms presuppose a sufficient relationship.  For example, "if it's a lemon, then it's sour," using modus ponens I say "here's a lemon, is it sour?", having a lemon is sufficient to cause sour.  I don't need anything with the lemon.  You can try modus tollens on your own.
2.  The two invalid forms presuppose a necessary relationship.  For example, "if it's a lemon, then it's sour," affirming the consequent I say "here's something sour, is it a lemon?," I'm assuming that the only way something could be sour is if it's a lemon, or that lemons are necessary for sour.  Try denying the antecedent on your own.
 
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III.  How does this apply to real-world research?  Consider obstacle detection by the blind.  Several explanations exist to explain how blind people can move around in unfamiliar environments without bumping into stuff.  The goal of the experiments is to see which of these is correct.
A.  Hypotheses:
1.  ESP:  Blind people have honed a sixth sense that we all possess but that sighted people ignore or can't use properly.
2.  Facial vision (cutaneous feedback):  Blind people are able to sense changes in air currents moving around their faces and use this information to avoid obstacles.
3.  Auditory:  Blind people pay more attention to sounds of things and use this information to avoid obstacles.
B.  Experiments:
1.  Experiments 1 - 3:  The goal is to establish the method.  Take two blind participants and two blindfolded sighted participants, position them in a hall, tell them to approach the wall.  Measure three things:  Distance perception (D-P):  How far away can they detect the wall?; Close perception (C-P):  how close can they get without hitting the wall?; and Number of collisions (Coll):  How many times do they hit the wall en route to getting close to it 25 times?  The experimenters used two basic conditions:  Hard shoes on a hard floor and socks on carpet.
Experiment 1 established that blind participants were good obstacle detectors, and that sighted participants weren't as good (but practice led to improvement).  This is illustrated in the “I.  Wall as obstacle” row in the table.
Experiment 2 refined the methodology by letting carpet runners guide the participants and using a movable screen as an obstacle.
Experiment 3 made one last change by using thicker carpeting.
The results of these experiments do three things.  First, they show that blind participants locate obstacles very well.  So, there's something to investigate with additional experiments.  Second, they establish a nice methodology for investigating obstacle detection.  Third, they show that sighted participants can become nearly as good as blind participants at detecting obstacles with just a little practice.  This pretty much eliminates the ESP hypothesis (it's unlikely that some latent sense could be activated so quickly).
2.  Experiment 4:  Is cutaneous feedback necessary?  The first step in chipping away at one of the hypotheses.  Repeat Experiment 3, but put a heavy felt hood over the participants' heads and heavy gloves on their hands.  Now, with no air currents, do the task.  You can see the data in the “IV.  Screen (felt head cover)” row of the table.  Participants were a bit worse, but not as bad as they should be if they must have air currents to detect obstacles.  Conclusion:  Cutaneous feedback isn't necessary for obstacle detection.
3.  Experiments 5 - 6:  But, air currents might still be sufficient.  In other words, both hypotheses could be correct.  To truly eliminate cutaneous feedback as an option we need to know:  Is cutaneous feedback sufficient?  To answer this, mask all sounds, and provide only cutaneous feedback.  In Experiment 5, this was done with big ear muffs, in Experiment 6 this was done with a masking tone.  Results:  Without auditory feedback, participants couldn't do it at all.  So, cutaneous feedback is not sufficient.  At this point, it's eliminated.  At the same time, we can say that auditory feedback is necessary (without it, no detection).
4.  Experiment 7:  To polish it up:  Is auditory feedback sufficient?  Put the participant in a sound-proof room with headphones that play the sounds from the hall.  Have the experimenter approach the wall, otherwise, the task is the same.  Now, all the participant gets is hearing.  The experimenter is actually doing the walking.  Result:  Auditory is sufficient.  This is the death blow for the other two hypotheses.  Auditory is necessary and sufficient, they're neither necessary nor sufficient.
 
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IV.  How does it all tie together?  Let's make a chart of the progress made in the series of experiments:
 
Experiment hierarchy
 
You can see how we started with multiple hypotheses (the first characteristic), performed an organized series of experiments (the second characteristic), and incremented knowledge by excluding alternatives (the third characteristic).  The steps are also all present (generate multiple hypotheses, design experiments, experiment and revise).  Notice how we repeated the steps several times to get a final answer.
One last note:  The wise student will use this technique for research projects as you're always guaranteed something to say after the results are in.  Also, introduction and discussion sections from this type of research pretty much write themselves.
 
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Research Methods Notes 13
Will Langston

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