Ch 10: Normal Distribution & Standard Scores Mon. Feb 23 rd, 2004.

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Presentation transcript:

Ch 10: Normal Distribution & Standard Scores Mon. Feb 23 rd, 2004

Normal Distribution (ND) wCommon theoretical distribution w/bell- shaped, symmetrical curve –Mean, median, mode all occur at peak wProperties of the ND: –Area under it = 1.0 (100% scores) –50% scores fall above midpt, 50% below –Betw Mean & + or - 1 SD = 34.13% of scores –Betw 1 & 2 SD (pos or neg) = 13.59% of scores –Beyond 2 SD (pos or neg) = 2.28% of scores –…or rule (68% between mean and +/- 1 SD; 95% betw mean and +/- 2 SD; 99.7% betw mean and +/- 3 SD.

Z scores wBy remembering these %s, if we have a ND, can determine % scores falling betw any score & the mean wUse of z scores (standardized scores) –The difference betw any score & the mean in standard deviation units Z = (y – ybar) / Sy –Notice sign & magnitude of z score…

(cont.) –If pos z score, raw score was above the mean; if neg z score  below the mean –Magnitude indicates how many standard deviations the score is away from the mean wSAT mean = 500, Sy = 100, your score (y) = 620 Z = 620 – 500 / 100 = 1.2 You scored 1.2 standard deviations above the mean Could compare to your friend, y = 540 Z = 540 – 500 / 100 =.4 (scored 4/10’s of standard deviation above mean)

Converting z to y wMay also need to transform z to raw score: Y = Ybar + Z(Sy) Someone tells you they scored 2 SD above the mean SAT; what raw score? Y = (100) = 700 SAT score

Standard Normal Table (aka Unit Normal Table) wTable showing proportion of scores corresponding to a certain z score wAppendix B in book: 3 columns –Col A gives z scores (all pos since symmetrical – just find same z for neg) –Col B gives proportion of scores betw the z score & mean –Col C gives proportion of scores beyond the z score

Using the table… wStart by sketching distribution, label mean & Sy, then score you’re interested in –Shade relevant area of distribution –Translate y into a z score, use table wExamples: IQ test has ybar=100, Sy = 15 wWhat is probability of having IQ 130? wWhat IQ score is needed to be in top 5%? wWhat is probability of having IQ between 90 &120?

Lab 11 wClick button for “ND” demo, move flags & find % associated w/scores wUse unit normal table (either link from lab or your book)…note how the tables differ slightly. wSPSS – lab11.sav dataset…create z scores (follow directions in lab)