Practical Testing Techniques
Verification and Validation
• Validation
– does the software do what was wanted?
• “Are we building the right system?”
– This is difficult to determine and involves subjective
judgments (reviews, etc.)
• Verification
– does the software meet its specification?
• “Are we building the system right?”
– This can only be objective if the specifications are sufficiently
– Implications?
• Everything must be verified
– …including the verification process itself
• Testing is a verification technique
– critically important for quality software
• Industry averages:
– 30-85 errors per 1000 lines of code;
– 0.5-3 errors per 1000 lines of code not detected before
• The ability to test a system depends on a
thorough, competent requirements document.
Goals of Testing
• Goal: show a program meets its
– But: testing can never be complete for nontrivial programs
• What is a successful test?
– One in which no errors were found?
– One in which one or more errors were
Goals of Testing - 2
• Testing should be:
– repeatable
• if you find an error, you want to repeat the test to show
• if you correct an error, you want to repeat the test to check
that you fixed it
– systematic
• random testing is not enough
• select test sets that are representative of real uses
• select test sets that cover the range of behaviors of the
– documented
• keep track of what tests were performed, and what the
results were
Goals of Testing - 3.
• Therefore you need a way to document
test cases showing:
– the input
– the expected output
– the actual result
• These test plans/scripts are critical to
project success!
• Errors of all kinds are known as “bugs”.
• Bugs come in two main types:
– compile-time (e.g., syntax errors)
which are cheap to fix
– run-time (usually logical errors)
which are expensive to fix.
Testing Strategies
• Never possible for designer to anticipate
every possible use of system. Systematic
testing is therefore essential.
• Offline strategies:
1. syntax checking & “lint” testers;
2. walkthroughs (“dry runs”);
3. inspections
• Online strategies:
1. black box testing;
2. white box testing.
Syntax Checking
• Detecting errors at compile time is preferable to
having them occur at run time!
• Syntax checking will simply determine whether a
program “looks” acceptable
• “lint” programs try to do deeper tests on code:
– will detect “this line will never be executed”
– “this variable may not be initialized”
• Compilers do a lot of this in the form of “warnings”.
• Formal procedure, where a team of
programmers read through code, explaining
what it does.
• Inspectors play “devils advocate”, trying to
find bugs.
• Time consuming process!
• Can be divisive/lead to interpersonal
• Often used only for critical code.
• Similar to inspections, except that inspectors
“mentally execute” the code using simple test
• Expensive in terms of human resources.
• Impossible for many systems.
• Usually used as discussion aid.
• Inspections/walkthroughs typically take 90120 minutes.
• Can find 30%-70% of errors.
Black Box Testing
• Generate test cases from the
– i.e. don’t look at the code
• Advantages:
– avoids making the same assumptions as
the programmer
– test data is independent of the
– results can be interpreted without knowing
implementation details
Consider this Function
function max_element ($in_array)
/* Returns the largest element in
return $max_elem;
A Test Set
3 16 4 32 9
9 32 4 16 3
22 32 59 17 88 1
1 88 17 59 32 22
9 6 7 11 5
5 11 7 6 9
561 13 1024 79 86 222 97
97 222 86 79 1024 13 561
• Is this enough testing?
“Black Box” Testing
• In black box testing, we ignore the internals of
the system, and focus on the relationship
between inputs and outputs.
• Exhaustive testing would mean examining
system output for every conceivable input.
– Clearly not practical for any real system!
• Instead, we use equivalence partitioning and
boundary analysis to identify characteristic
Black Box Testing
• Three ways of selecting test cases:
– Paths through the specification
• e.g. choose test cases that cover each part of the
preconditions and postconditions
– Boundary conditions
• choose test cases that are at or close to boundaries for
ranges of inputs
– Off-nominal cases
• choose test cases that try out every type of invalid input
(the program should degrade gracefully, without loss of
Equivalence Partitioning
• Suppose system asks for “a number between
100 and 999 inclusive”.
• This gives three equivalence classes of input:
– less than 100
– 100 to 999
– greater than 999
• We thus test the system against characteristic
values from each equivalence class.
• Example: 50 (invalid), 500 (valid), 1500
Boundary Analysis
• Arises from the observation that most
programs fail at input boundaries.
• Suppose system asks for “a number between
100 and 999 inclusive”.
• The boundaries are 100 and 999.
• We therefore test for values:
99 100 101
998 999 1000
lower boundary
upper boundary
White (Clear) Box Testing
• In white box testing, we use knowledge of the
internal structure to guide development of
• The ideal: examine every possible run of a
– Not possible in practice!
• Instead: aim to test every statement at least
White Box Testing
• Examine the code and test all paths
…because black box testing can never
guarantee we exercised all the code
• Path completeness:
– A test set is path complete if each path
through the code is exercised by at least
one case in the test set
White Box Testing - Example
if ($signal > 5) {
echo “hello”;
} else {
echo ‘‘goodbye’’;
• There are two possible paths through this
code, corresponding to $signal > 5 and
$signal <= 5.
• Aim to execute each one.
White Box Testing - Example 2
Consider an extremely simplified version of the code to authorize
withdrawals from an ATM:
$balance = get_account_balance($user);
if ($amount_requested <= 300) {
if ($amount_requested < $balance) {
} else {
echo “You don’t have enough to withdraw $amount_requested”;
} else {
echo “You can not withdraw more than $300”;
Mini-exercise: Document the white-box tests you need to create to
test this code snippet.
Weaknesses of Path
• Path completeness is usually infeasible
– e.g. for ($j=0, $i=0; $i<100; $i++)
if ($a[i]==True) $j=$j+1;
– there are 2100 paths through this program
segment (!)
… and even if you test every path, it
doesn’t mean your program is correct
Loop Testing
Another kind of boundary analysis:
1. skip the loop entirely
2. only one iteration through the loop
3. two iterations through the loop
4. m iterations through the loop (m < n)
5. (n-1), n, and (n+1) iterations
where n is the maximum number of
iterations through the loop
Test Planning
Test Planning
• Testing must be taken seriously, and rigorous
test plans and test scripts developed.
• These are generated from requirements
analysis document (for black box) and
program code (for white box).
• Distinguish between:
1. unit tests;
2. integration tests;
3. system tests.
Alpha and Beta Testing
• In-house testing is usually called alpha
• For software products, there is usually an
additional stage of testing, called beta testing.
• Involves distributing tested code to “beta test
sites” (usually prospective/friendly customers)
for evaluation and use.
• Typically involves a formal procedure for
reporting bugs.
Review: Triangle Classification
Question: How many test cases are enough?
Answer: A lot!
• expected cases (one for each type of triangle): (3,4,5), (4,4,5),
• boundary cases (only just not a triangle): (1,2,3)
• off-nominal cases (not valid triangle): (4,5,100)
• vary the order of inputs for expected cases: (4,5,4), (5,4,4), …
• vary the order of inputs for the boundary case: (1,3,2), (2,1,3),
(2,3,1), (3,2,1), (3,1,2)
• vary the order of inputs for the off-nominal case: (100,4,5),
(4,100,5), …
• choose two equal parameters for the off-nominal case: (100,4,4), …
• bad input: (2, 3, q)
• vary the order of bad inputs: …
Documenting Test Cases
• Describe how to test a
• Description must identify
– short description (optional)
– system state before executing the test
– function to be tested
– input (parameter) values for the test
– expected outcome of the test
Test Automation
• Testing is time consuming and repetitive
• Software testing has to be repeated
after every change (regression testing)
• Write test drivers that can run
automatically and produce a test report
Exercise: Create a Test Plan
• What are you going to test?
– functions, features, subsystems, the entire
• What approach are you going to use?
– white box, black box, in-house, outsourced,
inspections, walkthroughs, etc.
– what variant of each will you employ?
• When will you test?
– after a new module is added? after a change
is made? nightly? ….
– what is your testing schedule?
Exercise: Create a Test Plan - 2
• What pass/fail criteria will you use?
– Provide some sample test cases.
• How will you report bugs?
– what form/system will you use?
– how will you track them and resolve them?
• What are the team
– who will do what?

Testing Techniques