Having trouble w/ understanding pdf

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In summary, the conversation discusses the formula for calculating probability in a sampling scenario, where N represents the total number of items and n represents the number sampled. The formula is ((rCk) * (wCn-k))/(NCn), where r and w represent the number of red and white items, respectively. The formula is only valid for values of k between max{0, n – w} and min{n, r}, with a probability of zero otherwise.
  • #1
semidevil
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so I"m trying to understand pdf, the one where the formula is ((rCk) * (wCn-k))/(NCn). Here, xCx means x chooses x.

anyways, N is the total, and n is how much you sample out of it. and we know that r + w = N.

what is k?

I know this is diffenrent for all problems, but is there an analogy..I"m having trouble setting things up.
 
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  • #2
When you say, "the formula is ..." doesn't it strike you as odd that there is no "=" sign. What is a formula without that sign? What you have is an expression for the probability of getting k of the things which there are r of in total. If "r" represents the number of red balls in total, and "w" represents the number of white balls, then:

[tex]P(k) = \frac{{r\choose k} {w\choose {n - k}}}{{N\choose n}}[/tex]

However, this only holds for max{0, n – w} ≤ k ≤ min{n, r}. Clearly, the probability is zero otherwise.
 
  • #3


Hi there,

I understand that you are having trouble understanding the formula for calculating pdf, specifically the part where it involves choosing combinations (xCx). Let me try to break it down for you.

Firstly, pdf stands for probability density function, which is a statistical concept used to calculate the probability of a random variable falling within a certain range. In simple terms, it helps us understand the likelihood of a certain event occurring.

Now let's look at the formula: ((rCk) * (wCn-k))/(NCn). Here, r and w represent two different outcomes or categories, and k represents the number of times we choose the first outcome (r) out of n samples. So essentially, (rCk) means choosing k items from r items. Similarly, (wCn-k) means choosing n-k items from w items. This formula is used when we have two categories and we are interested in the probability of getting a specific number of outcomes from each category.

Moving on, N represents the total number of items in the sample, and n represents the number of items we are sampling from the total. So, N is the sum of all possible outcomes, which in this case is r + w.

Now, coming to your question about k - it represents the number of times we choose the first outcome (r) out of n samples. In other words, k is the number of events we are interested in from the first category (r) out of the total number of events (n). For example, if we are flipping a coin 10 times and we want to know the probability of getting 3 heads (r) out of the 10 flips (n), then k would be 3.

To summarize, k is the number of events we are interested in from the first category (r) out of the total number of events (n), and it can vary depending on the specific problem you are solving.

I hope this helps in understanding the formula and its components better. Remember, practice makes perfect, so try solving a few examples to get a better grip on it. All the best!
 

1. Why am I having trouble understanding PDF files?

There could be several reasons for this. One possibility is that you are not familiar with the software or application used to open PDF files. Another reason could be that the file you are trying to open is corrupted or damaged. It could also be that the content of the PDF is too complex or technical for you to understand.

2. How can I make it easier to understand PDF files?

There are several ways to make it easier to understand PDF files. One option is to use software or applications that allow you to annotate or highlight important information in the PDF. You can also try breaking down the content into smaller sections and taking breaks in between to process the information. Additionally, you can seek help from someone who is more familiar with the topic or material in the PDF.

3. Is there a specific way to read PDF files for better understanding?

Yes, there are certain techniques you can use to read PDF files for better understanding. One strategy is to skim through the document first to get an overall idea of the content. Then, you can go back and read more carefully, taking notes or highlighting important points. It may also be helpful to read out loud or summarize the information in your own words to improve comprehension.

4. Can I improve my understanding of PDF files through practice?

Yes, practice can definitely help improve your understanding of PDF files. The more you read and work with PDFs, the more familiar you will become with the format and how to navigate and comprehend the content. You can also try different techniques, such as using study guides or flashcards, to reinforce your understanding.

5. Are there any resources available to help me understand PDF files better?

Yes, there are many resources available to help you understand PDF files better. Some examples include online tutorials, user guides for PDF software, and forums where you can ask for help and advice from other users. You can also consult with a librarian or a tech-savvy friend for assistance. Additionally, there are tools and software that can assist with reading and understanding PDFs, such as text-to-speech converters and annotation features.

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