The hardest part of any testing is to determine how much testing is required. There is no fixed rule to dictate the length of testing needed. Even when the product has been tested there may still be something that's missed which could cause problems later.
It is impossible to test every product that is made. Not only would there be a limited time to do the testing but the costs involved would be astronomical.
It is impossible to test every product made.
Just as it's impossible to test every product that rolls off the production line, it also becomes impossible to test a product completely.
It is impossible to test a product completely.
Again this is due to the time available and the costs involved, making it just too impractical to completely test the product.
Consider a chocolate manufacturer who needs to make sure that their chocolates taste great. They have a production line that can produce 100,000 chocolate pieces a day. There are 20 people involved with managing the day to day production of the chocolates.
To completely test every chocolate piece made would mean taking a slice out of each chocolate to taste whether it was satisfactory and analysing the slice chemically to see whether it's constituents were as they should be.
However, by taking a slice from each chocolate piece manufactured, the chocolate would become unsellable. As the chocolate would be classed as being damaged.
The additional costs involved in employing additional people to taste every chocolate piece manufactured would substantially increase the manufacturing costs. Which in turn would increase the price of the chocolates. This in turn would prove too expensive for the end consumer.
The satisfactory recourse in this example would be to sample a random selection of chocolates, so when a new batch of chocolates is made, ten, twenty or thirty, for example, random pieces are taken to be analysed and tasted.
By doing this the costs of testing are reduced substantially. With little or no disruption in the manufacturing process for the chocolates.
By sampling, testing can still be effective as well as costing less to implement.
A large aircraft manufacturer must view their testing goals differently to that of the chocolate manufacturer. Whilst several factors will determine whether it may be too costly and impractical for the chocolate manufacturer to test all their chocolates. The aircraft manufacturer must test all the planes that they manufacture.
As each plane can cost several million pounds, testing each plane will be paramount to ensure that the customer receives not only value for money but a plane that is safe to fly.
Whilst every plane manufactured will be tested, the amount of testing done per plane will not be complete. Certain tests such as those involved with the tail strike will damage the plane and these tests will only be conducted on the initial batch of planes.
Even with a product with a small margin of error, such as a plane. There is still no effective way for it to be fully tested. As such there have been instances where this has led to a catastrophe.