【Machine Learning】【Andrew Ng】- Quiz2(Week 6)

1、You are working on a spam classification system using regularized logistic regression. “Spam” is a positive class (y = 1) and “not spam” is the negative class (y = 0). You have trained your classier and there are m = 1000 examples in the cross-validation set. The chart of predicted class vs. actual class is:

/	actual class:1	actual class:0
predicted class:1	85	890
predicted class:0	15	10

for reference:
- accuracy=(true positives + true negatives)/(total examples)
- precision = (true positives) /(true positives + false positives)
- recall = (true positives)/(true positives + false negatives)
- F1 score = (2*precision*recall)/(precision + recall)
What is the classier’s score (as a value from 0 to 1)?
答案：0.1581.
解析：
precision = 85/(85+890);
recall = 85/(85+15);

2、Suppose a massive dataset is available for training a learning algorithm. Training on a lot of data is likely to give good performance when two of the following conditions hold true. Which are the two?
A. When we are willing to include high order polynomial features of (such as x1^2,x2^2,x1x2,etc.).

B. We train a learning algorithm with a large number of parameters (that is able to learn/represent fairly complex functions).

C. The features contain sufficient information to predict accurately. (For example, one way to verify this is if a human expert on the domain can confidently predict y when given only x).

D. We train a learning algorithm with a small number of parameters (that is thus unlikely to overfit).

答案：BC。这两个条件都很重要，容易忘记。

3、Suppose you have trained a logistic regression classier which is outputing h(x).
Currently, you predict 1 if h(x)>= threshold, and predict 0 if h(x) less than threshold, where currently the threshold is set to 0.5.
Suppose you increase the threshold to 0.9. Which of the following are true? Check all that apply.
A、The classifier is likely to now have lower precision.
B、The classifier is likely to now have lower recall.
C、The classifier is likely to have unchanged precision and recall, but lower accuracy.
D、The classifier is likely to have unchanged precision and recall, but
higher accuracy.
答案：B。我理解得precision就是预测准的概率，recall就是不漏掉的概率。所以阈值越高，越容易预测准，越confident，但是也越容易有漏网之鱼。

4、Suppose you are working on a spam classier, where spam emails are positive examples (y=1) and non-spam emails are negative examples (y=0). You have a training set of emails in which 99% of the emails are non-spam and the other 1% is spam. Which of the following statements are true? Check all that apply.
A、If you always predict non-spam (output y=0), your classier will have a recall of 0%.
B、If you always predict spam (output y=1), your classier will have a recall of 0% and precision of 99%.
C、If you always predict spam (output y=1), your classier will have a recall of 100% and precision of 1%.
D、If you always predict non-spam (output y=0), your classier will have an accuracy of 99%.
答案：ACD
当全部预测为1时，如下：

/	actual class:1	actual class 0
predicted class:1	1%	99%
predicted class:0	0	0

accuracy = 1%
precision = 1%/(1%+99%) = 1%
recall = 1%/(1%+0) = 100%
当全部预测为0时，如下:

/	actual class:1	actual class 0
predicted class:1	0	0
predicted class:0	1%	99%

accuracy = 99%
precision = 0/(0+0) = 0%
recall = 0/(0+1%) = 0%

5、Which of the following statements are true? Check all that apply.
A、If your model is underfitting the training set, then obtaining more data is likely to help.
B、Using a very large training set makes it unlikely for model to overfit the training data.
C、It is a good idea to spend a lot of time collecting a large amount of data before building your first version of a learning algorithm.
D、After training a logistic regression classier, you must use 0.5 as your threshold for predicting whether an example is positive or negative.
E、On skewed datasets (e.g., when there are more positive examples than negative examples), accuracy is not a good measure of performance and you should instead use score based on the precision and recall.
答案：BE
A，错误，underfit主要是模型的问题，多点训练集并不会很大改善性能
B，对啊，训练集大了，模型自然很难满足所有的训练样本，所以也更难overfit啦
C，错误，第一个模型简单就好，不需要太多的数据，后面进行调整模型的时候需不需要更多的训练样本再看情况
D，错误，阈值取多少没有一个固定的数值，看你需要的性能，一般看F1值，取最大值。
E，正确。