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Using Excel Biostatistics 212 Lecture 4

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Housekeeping Questions about Lab 3? –replace vs. recode Final Project Dataset! –“Housekeeping” commands vs. data cleaning (don’t show data cleaning) A little short-handed today…

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Today... Why are we talking about spreadsheets? Pro’s and Con’s of using a spreadsheet for: –Data management, Statistics, Calculating, Modeling, Tables, Figures Cells Formulas Cutting and pasting formulas Spreadsheet style Examples

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Why spreadsheets? Excel is widely used, and for good reason –Store numbers and text –Calculations –Desktop graphics – Tables and Figures –Flexible creation of ledgers, models, other complex programs

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Why spreadsheets? How is a spreadsheet different than Stata’s data editor? –Less structured –Formulas –Formatting

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Why spreadsheets? How is a spreadsheet different than a database program like Access? –Less structured –Formula chains –Formatting

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Pro’s and Con’s of spreadsheets For data management –Pro’s Easy start – just name columns and start typing –Con’s No structure Can’t sort, filter or query data “Flat” file – no relational table structure allowed

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Pro’s and Con’s of spreadsheets For statistical analysis –Pro’s Easy start, if you know how to do formulas –Con’s Extremely limited range of options Difficult to document

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Pro’s and Con’s of spreadsheets For calculating, or “modeling” –Pro’s Repetitive calculations easy Complex calculations easy –Con’s Simple, 1-time calculations not as fast as a calculator Sometimes hard to decipher in retrospect

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Pro’s and Con’s of spreadsheets Tables and Figures – will discuss in Sessions 6 and 7

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Cells The basic building block of a spreadsheet Can contain: –Numbers –Text –Dates, times, other special formats –“blanks” Start with 46 million blank cells! (230 cols x 66536 rows x 3 worksheets)

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Cells, cont Enter anything you like into each cell (numbers, text, symbols, etc) using keyboard Contents displayed on spreadsheet Organized and named by column/row

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Formulas Use when you want the contents of one cell to depend on the contents of other cells ALWAYS starts with: = (an “equals sign”)

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Formulas Can contain: –Numbers –Text –References to cells –The usual math operators (+ - * / ^ ) –Built-in functions

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Formulas Cell contents update automatically when a referenced cell content changes “Chains” of formulas make for flexible calculating

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Formulas Contents of a cell displayed on spreadsheet The formula determining that content is displayed in the “formula box” Example

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Formulas Types of formulas –Arithmetic +, -, *, /, ^ –Logic IF(boolean, value 1, value 2) –Returns value 1 if TRUE, value2 if FALSE AND(boolean, boolean, boolean…) –Returns TRUE if all booleans are true, otherwise FALSE OR(boolean, boolean, boolean…) –Returns TRUE if any booleans are true, otherwise FALSE

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Formulas Types of formulas, cont –Functions, for example: SUM(range of cells) –Returns the sum of the values in the range –SUM(A5:A10) AVERAGE(range of cells) –Returns the average of the values in the range STDEV(range of cells) –Returns the standard deviation NORMINV(probability, mean of dist, SD of dist) –Returns the z-value associated with a given probability…

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Formulas Types of formulas, cont –Functions, for example: LN(number) –Returns the natural log of a number ABS(number) –Returns the absolute value of a number LEFT(text, number of characters=x) –Returns x number of characters from the text in the cell, starting at the left side… NOW() –Returns the current date, time

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Formulas Tips –Use parentheses IF(SUM(A5:A10)>5,1,IF(C9=“y”,2,3)) –Or do in multiple steps

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Cutting/Copying and Pasting Cutting and Copying treat formulas differently!

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Cutting and pasting formulas Excel assumes the cell references are ABSOLUTE, and you’re just moving the location of the formula cell Example

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Copying and pasting formulas Excel assumes the cell references are RELATIVE Example Shortcut: drag little square in the corner…

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Copying and pasting formulas If you want to FIX the position of a referenced cell, use $’s = A5 + $B$6 Example

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Examples Repetitive calculations –Back-transforming linear regression coefficients Complex calculations –2 x 2 template Modeling –Mortgage calculator –Risk integrator –Figure 2 for LDL-lowering paper

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Spreadsheet style Formatting –Text –Column width –Borders –Placement of stuff on the page

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Spreadsheet style For models: –Inputs on the left, in red –Outputs on the right, in blue, boxed, bolded, etc –Calculations on other sheets –“Protect” all cells besides inputs Format/Cells…/Protection Tools/Protect

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Take home points Understand cells and formulas Use copy/paste with and without fixed cells ($A$45) Good formatting adds significant value to your spreadsheet

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Lab 4 Practice with: –A repetitive calculation spreadsheet –A complex calculation spreadsheet –Introduction to making a figure with Excel Due before lecture next week Extra credit puzzle challenge – 2x2 excel template –Due Sept 18 th – email to mpletcher@epi.ucsf.edu

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To come… Next lecture –Epidemiologic analysis with Stata 2 x 2 tables, confounding and interaction Epitab commands Logistic regression introduction

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