Problem 205 - Applied Statistics and Probability for Engineers

Question

The CPU of a personal computer has a lifetime that is exponentially distributed with a mean lifetime of six years. You have owned this CPU for three years. (a) What is the probability that the CPU fails in the next three years? (b) Assume that your corporation has owned 10 CPUs for three years, and assume that the CPUs fail independently. What is the probability that at least one fails within the next three years?

Step-by-Step Solution

Answer

(a) 0.3935 (b) 0.9923

Step 1: Understanding the Exponential Distribution

The lifetime of a CPU is exponentially distributed with a mean of 6 years, implying a rate parameter \( \lambda = \frac{1}{6} \) year\(^{-1}\). The exponential distribution probability density function is given by \( f(t) = \lambda e^{-\lambda t} \).

Step 2: Calculating the Probability of Failure in Three Years

To find the probability that the CPU fails within the next 3 years given it has already survived 3 years, use the memoryless property of the exponential distribution. The probability is given by:\[P(T < 3) = 1 - e^{-\lambda(3)} = 1 - e^{-\frac{3}{6}} = 1 - e^{-0.5} = 0.3935\]

Step 3: Expanding to 10 Independent CPUs

When considering 10 CPUs, since the CPUs fail independently, we are interested in the event that at least one of these CPUs fails in the next 3 years. Let the probability of a single CPU failing in 3 years be \( p = 0.3935 \) and not failing be \( 1 - p = 0.6065 \). The probability that none fail is \( (0.6065)^{10} \).

Step 4: Calculating Probability of at Least One Failure

The probability that at least one CPU fails is the complement of the probability that none fail:\[P(\text{at least one fails}) = 1 - (0.6065)^{10} \approx 1 - 0.0077 = 0.9923\]

Key Concepts

Probability of Failure

The probability of failure in a defined time period is a common concept when dealing with issues of reliability and lifespan, especially in fields like engineering or computing. In the context of the exponential distribution, the probability of a device failing within a certain period can be computed by understanding the rate parameter and how it influences the distribution function. Specifically, if a CPU has an exponential lifetime with a mean of 6 years, the rate at which failures occur is given by the parameter \( \lambda = \frac{1}{6} \). This means on average, a CPU is expected to fail once every six years.
To calculate the probability that a CPU will fail within a specific timeframe, say 3 years, you use the cumulative distribution function (CDF) associated with the exponential distribution. The CDF, which gives the probability that a random variable is less than a certain value, is expressed as \( 1 - e^{-\lambda t} \). It tells us how likely it is for the CPU to not only experience a failure but also when that might occur within any given period.
In simpler terms, the longer an item has already lasted, the more likely it may fail soon, especially when no previous repair or replacement is involved within the predicted period, emphasizing the exponential nature of failure risks in electronic components.

Independent Events

When discussing independent events, it's important to understand that these are events where the outcome of one event does not affect the outcome of another. In the case of the CPU problems given, this concept is crucial when determining the likelihood of failure across multiple CPUs.
Suppose you have 10 CPUs in operation; if these CPUs fail independently, the probability that any particular CPU fails does not influence the probability of another's failure. Mathematically, this is expressed when probabilities of events are computed separately and then used in combination to determine the overall likelihood of those events occurring.
In practical terms, for each CPU, the probability of failure within the next 3 years remains \( p = 0.3935 \). The overall probability that at least one out of the 10 CPUs fails is calculated using the complement of the event that none fail. Thus, appreciated through the mathematical independence, each CPU's probability of functioning or failing is maintained without blending into the risks associated with others in the unit batch.

Memoryless Property

One of the most fascinating aspects of the exponential distribution is its memoryless property. This suggests that regardless of how long a process has been in effect, the future probability of failure for that process remains constant. In other words, the system "forgets" its past.
In the given exercise, even though a CPU has already been in operation for three years, the probability that it will fail in the next three years is the same as it would have been from the start for any three-year period. Mathematically, this means that the probability of failure in the next 3 years, given that the CPU has already been running successfully for 3 years, is still \( P(T < 3) = 1 - e^{-0.5} \), which is about 0.3935.
This property makes the exponential distribution particularly useful in modeling the time until an event, like a failure, occurs in systems where the past does not influence future outcomes. It allows for simplified calculations as there is no need to adjust for the "history" of the item's previous performance.

Complementary Probability

Complementary probability is a concept that aids in finding the probability of the opposite of an event occurring. It's crucial when assessing scenarios where knowing the likelihood of a non-event is easier or more useful than calculating the event directly.
Within our CPU problem context, we can use complementary probability to calculate the chance that at least one CPU fails in 10, after 3 years of operation. Knowing the probability of a single CPU not failing after 3 years is \( 1 - p = 0.6065 \), we can extend it to calculate the likelihood that all 10 CPUs will survive. Since events are independent, this is simply \( (0.6065)^{10} \).
The probability that at least one CPU fails is then the complement of this outcome: \( 1 - (0.6065)^{10} \), which is approximately 0.9923. This high probability reflects the tendency for at least one failure to occur in large batches of devices over time, underscoring the utility of complementary probability in risk assessment scenarios.