Saturday, July 14, 2012

How safe is a plastic bag?

A far-fetched example of the nature of safety and security

 Originally published at in Finnish on Fri 18th November 2011. The bottom line argument is added later.

..if you really have to buy one.

 When considering the safety of a plastic bag, it is good to approach the issue from two aspects: The purpose and the conditions of usage.

The common purpose of a plastic bag is to carry suitable items or to keep them under such condition that does not become harmful to the content of the plastic bag.

The Purpose


The purpose of the usage has a strong link with the objects of the action. This could be for instance carrying a bottle of milk home from the grocery (AKA functional requirement).

This action has quality objectives. You have to get the bottle home. And the milk should be kept cold until stored in the fridge. The ultimate objective (business requirement) of this action is to enjoy cold milk later, where the plastic bag has only a milestone objective - so it has only a support function to the enjoyable glass of milk. The safety (or security) needs of a plastic bag are sufficient tensile strength and paddings.

In most of the cases a plastic bag does not have any protection against heat or bounces. Those are conditional threats (they do not exist all the time), so we have to have handle them separated from the purpose (the time we want to carry milk).

The Conditions


The usage conditions have an effect on the safety. If you carry that milk bottle in a summer heat, it is quite sure that the milk is not drinkable after a while. Every 2 degrees in Celsius up halve the usage time. And drinking spoiled milk could be harmful to your health (safety). And it is nasty to your roommates too..

So the plastic bag does comply with the safety needs only in limited conditions. The business/functional requirement becomes a safety requirement in two cases:

  •     The safety of the ultimate action is not guaranteed in any other way
  •     The product could become harmful to its user in certain conditions
The plastic bag's safety objective (control objectives) from the conditions point of view is that it must maintain it's functional features in common conditions. You should not put it on the heater. It is also wise to guide the user to avoid the harmful conditions.



You can evaluate the plastic bag's safety from the point of the misuse. You can choke if you pull the plastic bag over your head and there are no other holes big enough. You can find the purpose (to pull the plastic bag over your head) and the conditions (no other holes) form the previous sentence. Those are only described as threats.

Should we ban the carriage of a bottle of milk on a crisp fall eve only because someone could get choke on the plastic bag?


As a summary..


If you are concerned about the security of [anything], do not handle it apart from the essential action. There are many security details, for and against. You should estimate them from the points of the purpose, usage and misuse. Use the very same estimating tools and methods that you use in other decision-making - for instance in investments. What do we gain, what we could lose, how to guarantee the benefits and how to protect against the stranded investments?

Compare the need for freedom of speech with the need for cencorship to protect internet users from unpleasant content. Are the protective measurements justified or do they turn against the purpose?

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