Using Excel Biostatistics 212 Lecture 4

Housekeeping Finish Lab 2 today and/or start Lab 3 Mac Addendum Copying and pasting from Stata

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

Why spreadsheets? Excel is very popular, and very useful –Store numbers and text –Calculations –Desktop graphics – Tables and Figures –Flexible creation of ledgers, models, other complex programs

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

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

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 or query data Can’t generate new variables “Flat” file – no relational table structure allowed

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

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 Sometimes hard to decipher in retrospect

Pro’s and Con’s of spreadsheets Tables and Figures – will discuss in Sessions 5 and 6

Cells The basic building block of a spreadsheet Can contain: –Numbers –Text –Dates, times, other special formats –“blanks” start with 230 x = 15.3 million blank cells

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

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

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

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

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

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

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…

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

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

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

Copying and pasting formulas Excel assumes the cell references are RELATIVE Shortcut (drag little square) Example

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

Examples Repetitive calculations –Back-transforming linear regression coefficients Complex calculations –2 x 2 template Modeling –Mortgage calculator –Risk integrator

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

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

Summary Know what Excel can do for you It’s all about formulas

Lab 3 Practice with: –A repetitive calculation spreadsheet –A complex calculation spreadsheet –Introduction to making a figure with Excel Extra credit puzzle challenge – 2x2 excel template Due 10/25/05

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