Original subtitles

- Hi, I'm William Lidwell

and this is Universal Principles of Design.

In this movie, Garbage In-Garbage Out,

The Science of Taking Out the Trash.

On December 11, 1998, NASA launched

the Mars Climate Orbiter,

the first robotic probe with a mission

of studying climate change on another world.

The flight to Mars was uneventful,

requiring occasional course corrections

but nothing noteworthy.

Then on September 8, 1999,

a trajectory correction maneuver

was transmitted from Earth to the probe

to put it on the optimal path for orbital insertion.

On September 23, 1999, NASA

lost communication with the probe

and soon thereafter discovered

that it had disintegrated in the Martian atmosphere.

The cause, trajectory corrections were entered

in English units versus the required metric units,

bringing it too close to the planet

where it burned up in the atmosphere.

The Mars Climate Orbiter is a tragic demonstration

of the principle of garbage in-garbage out,

sometimes referred to by the acronym GIGO.

What is garbage in-garbage out?

The phrase means that the quality of system output

is largely dependent on the quality of system input.

Simply stated, bad input gives you bad output.

The Mars Climate Orbiter received bad input

and the output destroyed the probe.

The garbage in metaphor refers to two categories

of input problems, type and quality.

Problems of type occur when the wrong type

of input is provided.

For example, entering a phone number

into a credit card number field.

Or say, entering data in English versus metric units.

Problems of quality occur when the correct

type of input is provided but with defects.

For example, entering a phone number

into a phone number field

but entering it incorrectly.

Or say, a pilot pulling the control stick

to perform a maneuver that would

cause the airframe to fail.

It's the right kind of data,

just out of a safe range.

Well-designed systems deal with

the garbage in-garbage out problem in four ways.

One, prevent bad input.

The preferred method of preventing GIGO

is verifying that input is correct

before it enters a system.

For example, on a web form,

it would mean strong data validation

on all fields, right kind of data

in the right kind of fields

in the proper format before it can be submitted.

Two, correct bad input.

If garbage input still gets through,

second best is to detect and correct it

at the system level before acting on the data.

For example, many systems run scheduled processes

to clean up data, checking for duplicates

and valid addresses and proper formats, etc.

Three, prevent bad output.

If garbage input gets by one and two,

third best is to detect and prevent

output before it occurs.

For example, a preview lets a person see

what the output will look like before it occurs,

giving them an opportunity to verify

that it's correct and abort if necessary.

And four, correct bad output.

If garbage input gets by one through three,

then the last ditch option is to detect

and correct bad output after it occurs.

Well-designed systems minimize

the effort required to do this.

For example, including a prepaid return shipping label

with orders placed online.

So whether you use your knowledge

of garbage in-garbage out to reduce

input errors on your website,

to increase the reliability and safety

of planes, trains, and automobiles,

or to prevent a future space probe

from assuming a catastrophic trajectory,

remember the science of taking out the trash.