When a product fails- be it a wrist watch, a mobile phone, an electrical inverter or motor car; the first thing that comes to mind is that it's a manufacturing or inspection error. However it could be neither, the problem could be an inherent design problem; and the term used to define such problems is "Reliability".
The reliability can be defined as the probability that an item will adequately perform its specified purpose for a specified period of time under specified environmental conditions. I think it is important for all viz., manufacturers, providers of service and users of products and service to understand this concept, to enable problem resolution. If the problem is not recognized as a reliability problem, the actions taken to resolve the same may not yield any results. For example, my wrist watch, which I have mentioned in my earlier articles. Each time I return my watch for the random problem of "slow running", the service centre sends the watch for an overhaul and returns the same. They are actually unable to see the problem because the occurrence is random and not very frequent. They fail to realize that it is to do with the reliability of the design of the product. The PDA phone I possess has wonderful features from Wi-Fi and GPS to email synchronization; I like the way I am able to synchronize the folders in my web mail like Gmail, from which I would like to download messages. However I am not completely sure that all my mails actually get's pushed onto my phone; there could be an interruption in the service anytime and at times the system gets corrupted and the entire setting has to be reprogrammed to revive smooth functioning. Though the phone has very good features, there is a question of reliability of the same. So we may say good "Quality" but poor "Reliability"; though broadly reliability is also considered a method of measuring quality.
Quality relates to meeting requirements or performing the intended function when first manufactured or when first purchased by the customer. It has no relationship to product performance over time or under any given conditions. Reliability relates to the performance of the product over time and under a specified set of conditions.
If the customer perceives that the product is reliable, they are more likely to buy it; on the other hand one of the outcomes of an unreliable product is more warranty claims. We measure quality typically by the number of defectives retuned by the customer as a ratio of the parts shipped or the number of lots rejected as a ratio of the shipments made; and this is generally expressed in PPM ( parts per million). Similarly it is important to be aware how reliability can be quantified. Some common methods to look at reliability are as follows:
1. Mean time between failures ( MTBF)- This is the mean time between successive failures of a repairable product
2. Failure Rate- is the number of failures per unit time
3. Mean time to Failure ( MTTF)- is the mean time to failure of a non-repairable product or mean time to first failure of a repairable product
4. Mean Life- is the mean value of life ( "life" may be related to major overhaul, wear-out etc)
5. Mean time to first failure ( MTFF)- is the mean time to first failure of a repairable product
6. Mean time between maintenance (MTBM) - is the mean time between a specified type of a maintenance action.
It is important to remember that MTBF, a commonly used term, is not the same as "operating life", "service life" or other indices which generally connote overhaul or replacement time; and an increase in MTBF does not result in a proportional increase in reliability i.e., the "probability of survival".
|Reliability relates to the performance of the product over time and under a specified set of conditions|
If the MTBF of a product is known, say 25,000 hours, than the failure rate is the inverse of that (1/ 25,000)= .00004 failures / hour. Based on the parts produced per year and the total operating time, it would be possible to calculate the return rate. Further calculations and analysis on reliability is beyond the scope of this article; however I do hope that this gives a fair idea of the concept of reliability which is important to both the manufacturer (and provider of service) and the user. . And I am sure by now you're trying to guess the mean time between calls being dropped while talking on your mobile phones of recent.
Pradeep Kumar E.T. A Master Black Belt in Six Sigma , is the Country Manager- Operational Excellence with Tyco Electronics Corporation India Pvt Ltd. Feedback can be e- mailed to firstname.lastname@example.org
Issue BG96 Mar09