How much quality is good enough?

Imagine if somebody says 'The quality of a product is 99% defect free'. Seems like WOW!! isn't it?

Think again. In term of deviation this (99% accuracy) stands at around 4 Sigma which means around 6000 defects per million opportunities. Now consider this:

3 Sigma is 93.3% accuracy - 66807 defects per million opportunities
4 sigma is 99.38% accuracy - 6210 defects per million opportunities
5 sigma is 99.97% accuracy - 233 defects per million opportunities
6 sigma is 99.99966% accuracy - 3.4 defects per million opportunities
7 sigma is 99.9999981% accuracy - 0.019 defects per million opportunities

Now let's compare some data:

1. If we send 3 Million letters to the users that means
1. With 4 sigma we will lose 3000 letters
2. With 6 sigma we will lose only 1
2. For every week of TV broadcasting per channel
1. With 4 sigma 1.7 hours of dead air broadcast
2. With 6 sigma 1.8 seconds of dead air broadcast
3. Out of every 500,000 computer restarts
1. With 4 sigma around 4000 system crashes
2. With 6 sigma Less than 2 system crashes 
4. Total flight canceled in US every 3 weeks
1. With 3 sigma 964 flights cancellations in a day
2. With 4 sigma 30 flights cancellations
3. With 6 sigma 1 flight cancellations

Many of the spaceship and medical advancement projects runs at more than 7+ sigma quality standard as there are no scope whatsoever, for errors, due to the level of cost, time and importance surrounding those projects.

Amazingly, mumbai dabbawalas stands at a whopping 7+ sigma (only 1 defect in 6 Million opportunities). This is a really amazing, considering that most of the workers (5000 workers and 170 thousand lunch boxes a day) are uneducated and uses only a color code system to identify lunch boxes. Who says to be high quality processes it needs to be complex. Instead it just needed to be simple and effective!! Isn't it?

Currently we do not see many IT organizations implementing Six Sigma level processes and only very few organizations implementing this practice. In future we might see such level of quality implemented in IT projects but that may be some time, from now. Many organizations desist from this practice as short term investment might be high and returns are primarily long term only.

BTW, you can also get sigma values for some of the things around you e.g.: 'Number of defects per million line of code' or 'Number of defect you found per thousand test steps' and check the sigma value for the same...Have fun.