371chapter6 - Chapter 6 Bernoulli Trials Revisited 6.1 Is p...

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Chapter 6 Bernoulli Trials Revisited 6.1 Is p Constant? Recall that there are three assumptions to BT. The first, that each trial yields a dichotomy, is easy to verify or refute. The other two assumptions can neither be verified nor refuted with certainty. In this Section we present methods for investigating the second assumption, that the probability of success remains constant. Our methods will consist of informal descriptive techniques and formal tests of hypotheses. Both of these categories of methods are very useful to a scientist. I begin with an extended example of data I collected circa 1990 on Tetris. Tetris is a video game (I fear that this expression—video game—is hopelessly dated; perhaps you can give me a better name to use for the next version of these notes!) that can be played on a variety of ‘systems.’ If you have never played Tetris, then the following description of the game may be difficult to follow. Tetris rewards spatial reasoning and, not surprisingly, lightning reflexes. Essentially, one of seven possible geometrical shapes of four blocks ‘falls’ from the top of the screen. The player translates and/or rotates the falling shape in an attempt to complete, with no gaps, a horizontal row of blocks. Each completed row of blocks disappears from the screen and blocks above, if any, drop down into the vacated space to allow room for more falling shapes. A player’s score equals the number of rows completed before the screen overflows, which means that the current game is over. After every ten completed lines the speed of the falling shapes increases making the game more difficult. Each drop of a shape by the computer can be viewed as a trial. Each trial has seven possible outcomes. For now, the shapes are divided into two types: a straight row (my favorite, labeled a success) and any other shape (a failure). I observed 1,872 trials during eight games of Tetris on my son’s Nintendo System. The methods I present below are based on taking the total number of trials, 1,872 in the current case, and dividing them into two or more segments. The choice of segments is a matter of taste and judgment and unless I have a good reason to do otherwise, I advocate dividing the data into segments of equal or nearly equal length, and I usually advocate having two segments of trials. For these Tetris data, however, there is a structure that I want my analysis to reflect, namely, every time a game ends the system is ‘reset’ before the next game. (There was literally a button that said ‘Reset.’) Thus, 59
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Table 6.1: A Comparison of Eight Games of Tetris. Frequencies Row Proportions Game S F Total S F Total First 22 171 193 0.114 0.886 1.000 Second 33 185 218 0.151 0.849 1.000 Third 36 215 251 0.143 0.857 1.000 Fourth 25 206 231 0.108 0.892 1.000 Fifth 30 198 228 0.132 0.868 1.000 Sixth 42 220 262 0.160 0.840 1.000 Seventh 33 215 248 0.133 0.867 1.000 Eighth 33 208 241 0.137 0.863 1.000 Total 254 1618 1872 0.136 0.864 1.000 Figure 6.1: Plot of Proportion of Success for Eight Games of Tetris.
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This note was uploaded on 10/23/2009 for the course STAT STATS 371 taught by Professor Professorwardrop during the Fall '09 term at Wisconsin.

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371chapter6 - Chapter 6 Bernoulli Trials Revisited 6.1 Is p...

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