Transcript Introduction to the Class, and to the program

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
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Cells
Formulas
Cutting and pasting formulas
Spreadsheet style
Examples
Why spreadsheets?
• Excel is very popular, and very useful
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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
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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:
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Numbers
Text
Dates, times, other special formats
“blanks”
• start with 230 x 66536 = 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:
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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
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Text
Column width
Borders
Placement of stuff on the page
Spreadsheet style
• For models:
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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