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The Instat Guide to Choosing and Interpreting Statistical Tests

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The Instat Guide to Choosing and Interpreting Statistical Tests Statistics for biologists The InStat Guide to Choosing and Interpreting Statistical Tests GraphPad InStat Version 3 for Macintosh By GraphPad Software, Inc. © 1990-2001,,, GraphPad Soffftttware,,, IIInc... Allllll riiighttts reserved... Program design, manual and help Dr. Harvey J. Motulsky screens: Paige Searle Programming: Mike Platt John Pilkington Harvey Motulsky Maciiintttosh conversiiion by Soffftttware MacKiiiev... www...mackiiiev...com Project Manager: Dennis Radushev Programmers: Alexander Bezkorovainy Dmitry Farina Quality Assurance: Lena Filimonihina Help and Manual: Pavel Noga Andrew Yeremenko InStat and GraphPad Prism are registered trademarks of GraphPad Software, Inc. All Rights Reserved. Manufactured in the United States of America. Except as permitted under the United States copyright law of 1976, no part of this publication may be reproduced or distributed in any form or by any means without the prior written permission of the publisher. Use of the software is subject to the restrictions contained in the accompanying software license agreement. How ttto reach GraphPad: Phone: (US) 858-457-3909 Fax: (US) 858-457-8141 Email: [email protected] or [email protected] Web: www.graphpad.com Mail: GraphPad Software, Inc. 5755 Oberlin Drive #110 San Diego, CA 92121 USA The entire text of this manual is available on-line at www.graphpad.com Contents Welcome to InStat........................................................................................7 The InStat approach ---------------------------------------------------------------7 What InStat does-------------------------------------------------------------------7 How to start-------------------------------------------------------------------------8 Introduction to statistical principles .............................................................9 When do you need statistical calculations? -----------------------------------9 The key concept: Sampling from a population-------------------------------9 Confidence intervals------------------------------------------------------------- 12 P values ---------------------------------------------------------------------------- 13 Hypothesis testing and statistical significance ------------------------------ 15 The Gaussian distribution and testing for normality...................................19 What is the Gaussian distribution?-------------------------------------------- 19 What’s so special about the Gaussian distribution? ----------------------- 19 Nonparametric tests ------------------------------------------------------------- 20 Transforming data to create a Gaussian distribution ---------------------- 21 Testing for normality------------------------------------------------------------- 22 Tutorial: The InStat approach.....................................................................25 Step 1. Choose data format ---------------------------------------------------- 25 Step 2. Enter data ---------------------------------------------------------------- 27 Step 3. Summary statistics ------------------------------------------------------ 28 Step 4. Select a statistical test -------------------------------------------------- 29 Step 5. View the results --------------------------------------------------------- 30 Step 6. On your own ------------------------------------------------------------ 30 Descriptive statistics ..................................................................................31 Column statistics ----------------------------------------------------------------- 31 Entering averaged data into InStat -------------------------------------------- 33 One sample tests........................................................................................35 Introduction to one sample tests---------------------------------------------- 35 Choosing the one-sample t test or Wilcoxon test-------------------------- 35 The results of a one-sample t test --------------------------------------------- 36 The results of a Wilcoxon test ------------------------------------------------- 40 Comparing two groups (t tests etc.)............................................................43 Introduction to t tests------------------------------------------------------------ 43 Entering t test data into InStat --------------------------------------------------43 Choosing a test to compare two columns------------------------------------45 The results of an unpaired t test------------------------------------------------47 The results of a paired t test-----------------------------------------------------53 The results of a Mann-Whitney test -------------------------------------------58 The results of a Wilcoxon test --------------------------------------------------61 Comparing three or more groups (one-way ANOVA, etc.) ..........................65 Introduction to ANOVA ---------------------------------------------------------65 Entering ANOVA data into InStat ----------------------------------------------65 Choosing a one-way ANOVA analysis ----------------------------------------67 The results of one-way ANOVA------------------------------------------------70 The results of repeated measures ANOVA-----------------------------------77 The results of a Kruskal-Wallis test---------------------------------------------81 The results of a Friedman test --------------------------------------------------84 Contingency tables.....................................................................................87 Creating contingency tables ----------------------------------------------------87 Analysis choices for contingency tables---------------------------------------89 Results of contingency table analyses -----------------------------------------91 Linear regression and correlation...............................................................97 Introduction to linear regression and correlation ---------------------------97 Entering data for correlation and linear regression -------------------------98 Choosing linear regression or correlation ------------------------------------99 Results of correlation----------------------------------------------------------- 101 Results of linear regression ---------------------------------------------------- 103 Reading unknowns from standard curves ---------------------------------- 106 Multiple regression and correlation..........................................................109 Introduction to multiple regression and correlation---------------------- 109 Entering data for multiple regression and correlation-------------------- 111 Analysis choices for multiple regression and correlation ---------------- 112 Interpreting a correlation matrix --------------------------------------------- 113 The results of multiple regression-------------------------------------------- 113 Using InStat..............................................................................................121 Installing GraphPad InStat----------------------------------------------------- 121 Online Help---------------------------------------------------------------------- 122 Importing and exporting data ------------------------------------------------ 123 Working with the data table -------------------------------------------------- 127 Arranging data------------------------------------------------------------------- 131 Selecting columns to analyze -------------------------------------------------131 After you view the results------------------------------------------------------132 Menus and toolbars ------------------------------------------------------------134 InStat Preferences---------------------------------------------------------------137 InStat files-------------------------------------------------------------------------138 GraphPad Software...................................................................................139 Software license agreement---------------------------------------------------139 Technical support---------------------------------------------------------------140 The GraphPad web site--------------------------------------------------------141 Citing GraphPad InStat---------------------------------------------------------141 What’s new in InStat version 3? ----------------------------------------------141 GraphPad Prism-----------------------------------------------------------------142 Intuitive Biostatistics (book) ---------------------------------------------------143 Index ........................................................................................................145 Welcome to InStat The InStat approach GraphPad InStat is designed to help the experimental or clinical scientist analyze small amounts of data. Although InStat can do some data manipulation and selection, it is not designed to manage a large database with many variables. InStat works best when you have a single question in mind. Enter the data to answer that question, and InStat guides you to pick the right test to answer it: • The first step – even before entering or importing data – is to tell InStat what kind of data you wish to enter. InStat will then present you with a data table for that kind of data. • The next step is to enter data. You can type in numbers, paste from the clipboard, or import a text file. • InStat then asks you several questions to choose a test. On-line help (or this manual) can help you answer those questions. • Finally InStat presents the results, avoiding jargon when possible. On-line help, and this manual, can help you interpret the values. This manual
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