sas proficiency test by judy loren

SAS Proficiency Test

Judy Loren August 24, 2007

SAS Proficiency Test Used as a hiring tool years ago Publishers Clearinghouse Sensitive but not specific If you pass it doesnt mean youre good But if youre good, you should pass

Copyright Health Dialog Services Corporation 2007. All rights reserved.

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Part I Question 1_1 What happens if no VAR statement is used in PROC PRINT? All variables

What order? In the order the vars exist on the dataset

What determines that? In the order it encounters the variable names during syntax scan

Need to understand how SAS processes a data step Demonstration spt_1_1.sas


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Part I Question 1_2 Explain what a (data step) SELECT statement does. Sets up a series of IFTHEN.ELSE logic based on the value of 1 variable.

Demonstration spt_1_2.sas


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Part I Question 1_3 What is the default length of a numeric variable?8

All numbers are floating point binary, NOT a sequence of base 10 digits. Binary joke: There are 10 kinds of people in the world: those who understand binary, and those who dont. Each byte has 8 bits, which can store an integer as high as 27 or 128. Two bytes, 16 bits, 215 or 32,768. Since all numbers are floating point, SAS uses some bits for the power and the sign, so it takes a minimum of 3 bytes to store any number at all.Copyright Health Dialog Services Corporation 2007. All rights reserved. 5

Part I Question 1_3 Largest integer SAS can store accurately by length of numeric variable:



Length 2 3

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Part I Question 1_4 What is the difference between DO WHILE and DO UNTIL? WHILE stops when the condition is false; UNTIL stops when the condition is true WHILE evaluates at the TOP of the loop UNTIL evaluates at the BOTTOM of the loop



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Part I Question 1_5 What does BY after SET A; do? Creates first. and last. variables for each var in the BY statement ERRORs if the dataset is not sorted by the vars in the BY statement

What does BY after SET A B; do? In addition to above, interleaves the observations from A and B so that the resulting dataset remains sorted by the BY vars Can result in problems if A and B have different variables.



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Part I Question 1_6 What is a subsetting IF statement? Condition for keeping observations Causes that execution of the data loop to end if condition is false Can cause problems When used in combo with IF FIRST. or IF LAST. When used in combo with DO / SET / END logic



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Part I Question 1_7 Whats the difference between INFILE and INPUT? INFILE declares which file will supply records INPUT translates portions of each record into variables in an observation Both are executable INPUT reads from the most recently declared INFILE Can change INFILES during processing Attempt to read an additional record from any INFILE that is at the bottom will cause the DATA step to end How to concatenate INFILEs



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Part I Question 1_8 What information is given by a Proc Contents? Header information only

Cannot give observations count of a view. Why? How do you modify the header? Proc datasets

Which elements can you change? Names, labels, formats

Which elements can you NOT change without re-creating the dataset? lengths

Demonstration spt_1_8.sasCopyright Health Dialog Services Corporation 2007. All rights reserved. 11

Part I Question 1_9 How would you write an input statement to read a character variable X of length 1 from column 2? Input @ 2 X $1.; Input X $ 2-2; Input +1 X $1.; Other?



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Part I Question 1_10 What is the purpose of the single trailing @ sign? Hold the current line in the input buffer for additional inputs Used to check variables to decide: Whether to read the rest of the line What format to use on the rest of the line



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Part I Question 1_11 What statements are used to include or exclude specific variables in a data step? Statements drop, keep Dsoptions (keep= ), (drop= ) On incoming dataset or output dataset Interactions with rename Shortcuts



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Part I Question 1_12 How can you generate a 2-dimensional crosstabulation? Proc Freq Proc Tabulate Proc Report



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Part I Question 1_13 In the macro language, all macro commands begin with?% Macro variables requiring substitution start with &



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Halftime


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Part II Question 2_1 What occurs if multiple datasets are included in a SET statement? concatenation

Already addressed in 1_5


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Part II Question 2_2 What symbol is used to concatenate two character values? || as in a = trim(b) || left(z); New function CATX will do the trim, left, and insert a separator a = CATX( ,b,z);

CAT CATS CATT CATX

Concatenates character strings without removing leading or trailing blanks Concatenates character strings and removes leading and trailing blanks Concatenates character strings and removes trailing blanks Concatenates character strings, removes leading and trailing blanks, and inserts separators


Part II Question 2_3 Explain the substr function. Ray Cloutiers talk on 2/13 Left or right of = Isolates certain positions within a character variable

New functions Char(name,3) takes just 1 character Third = char(name,3); First(name) takes the first character Initial = first(name);



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Part II Question 2_4 What are the differences among these functions? Round() Ceil() Floor()



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Part II Question 2_5 In the data step, records are usually added to a SAS dataset automatically. Under what circumstances is there no automatic output? IKWID



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Part II Question 2_6 How might the result of these two statements differ? C = A + B; C = SUM ( A , B ) ; Missing values

What is the result of this statement? C = SUM ( A1 A10 );



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Part II Question 2_7 In a data step, how can you identify which of 2 input datasets an observation came from? Set one (in=a) two (in=b); If a and b;

What happens if you use the same in= variable for 2 datasets? Demonstration spt_2_7.sas


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Part II Question 2_8 How do you begin a data step if you do not want to create a SAS dataset? Data _null_;

When would you use this? Create a flat file (usually use export) Create a report (usually use export) Load a macro variable (often use SQL) New function call symputx handles left justification, trimming and character conversion



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Part II Question 2_8 Call symputx 3rd argumentNOT case sensitive G specifies that the macro variable is stored in the global symbol table, even if the local symbol table exists. L specifies that the macro variable is stored in the most local symbol table that exists, which might be the global symbol table. F specifies that if the macro variable exists in any symbol table, CALL SYMPUTX uses the version in the most local symbol table in which it exists. If the macro variable does not exist, CALL SYMPUTX stores the variable in the most local symbol table.Copyright Health Dialog Services Corporation 2007. All rights reserved. 26

Part II Question 2_9 What are the automatic variables first. and last. ? Created whenever BY is used in data step First.patkey is true when this observation is the first one with this value of patkey, false otherwise Last.patkey is true when this observation is the last one with this value of patkey, false otherwise When multiple variables appear in the BY statement, these variables operate within levels of the variables above them.

Useful for finding duplicates Demonstration spt_2_9.sas


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Part II Question 2_10 What does a 2 level input dataset name such as OLD.TWO indicate ? TWO is a dataset in the library with the libname OLD

What is the default libname, where one-level dataset names go? work %savework allows you to save your work space



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Part II Question 2_11 What statement allows you to keep track of previous values of variables or keep a running total? Retain a 0; initializes a to 0 the first time through the data step a+1; this form will automatically retain a What about a+b; Treats missing values of b as zero a+ (-b); decrements by b NOT a - b;

What happens if you retain the value of a variable in the incoming dataset? Demonstration spt_2_11.sasCopyright Health Dialog Services Corporation 2007. All rights reserved. 29

Part II Question 2_12 When an invalid data field is encountered when inputting a numeric variable, what happens? Variable set to missing (or whatever value is specified in the INVALIDDATA system option) Message written to log Message written to log Message written to log _error_ set to 1 (dump buffer and all variable values)

What are some ways to avoid this? ? or ?? modifier Both suppress the invalid data message ?? Sets _error_ to 0


Part II Question 2_13 Whats the difference between $w. and $CHARw. informats? $w. will cause the characters found to be left justified within the variables value $CHARw. preserves leading blanks



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Part III Question 3_1 Explain the _type_ variable generated by proc summary. Another application of binary numbersProc summary; class _type_ sex 22 0 0 0 0 1 1 1 1 agecat 21 0 0 1 1 0 0 1 1 plan ; 20 0 = 0 1 = 1 0 = 2 1 = 3 0 = 4 1 = 5 0 = ? 1 = ?

(grand total) (plan only) (agecat only) (plan & agecat) (sex only) (sex and plan)


Part III Question 3_3 What is the purpose of a %GLOBAL statement? To make a macro variable common to all macros



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Part III Question 3_4 What do CALL SYMPUT and CALL SYMGET do? Store and retrieve values of macro variables

Values only exist after data step is completed Ive never had occasion to use SYMGET Demonstration spt_3_4.sas


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Part III Question 3_5 If you want to print a hyphenated phone number with an area code in parentheses, how would you do it? Bunch of substr OR PICTURE



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Part III Question 3_8 What date is the reference date for calculating the value of a SAS date variable? January 1, 1960 What is the value of A in the following? (write these down!) A = 01jan1960d; B = mdy (1, 2, 1960); C = intnx(month, A, -1 ); D = intck(year, A, C);



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Part III Question 3_9 This is the question for all the money. This is the question that separates the pros from the amateurs. This is the question that makes you an ubergeek or just a wannabe. What is _n_? NOT an observation counter Number times youve executed the data loop



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Game Over How did you do?? Did you learn anything?? I did!


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sas proficiency test by judy loren

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