# 吴恩达深度学习学习笔记——C3W2——机器学习策略2——练习题

2021/02/08 07:10

### C3W2 Quiz - Autonomous driving (case study)

Ans: B

Note: As discussed in lecture, applied ML is a highly iterative process. If you train a basic model and carry out error analysis (see what mistakes it makes) it will help point you in more promising directions.

Ans: False

Note: Softmax would be a good choice if one and only one of the possibilities (stop sign, speed bump, pedestrian crossing, green light and red light) was present in each image.

Ans: D

Ans: False

Note: As seen in the lecture on multi-task learning, you can compute the cost such that it is not influenced by the fact that some entries haven’t been labeled.

Ans: A

Note: As seen in lecture, it is important that your dev and test set have the closest possible distribution to “real”-data. It is also important for the training set to contain enough “real”-data to avoid having a data-mismatch problem.

Ans: B、D

Ans: C

Note: To get an idea of this, we will have to measure human-level error separately on both distributions. The algorithm does better on the distribution data it is trained on. But we do not know for certain that it was because it was trained on that data or if it was really easier than the dev/test distribution.

Ans: C

Ans: A

Ans: B

Ans: A

Ans: A

Ans: D

Note: The problem he is trying to solve is quite different from yours. The different dataset structures make it probably impossible to use transfer learning or multi-task learning.

Ans: False

Ans: A

1. To help you practice strategies for machine learning, in this week we’ll present another scenario and ask how you would act. We think this “simulator” of working in a machine learning project will give a task of what leading a machine learning project could be like!

You are employed by a startup building self-driving cars. You are in charge of detecting road signs (stop sign, pedestrian crossing sign, construction ahead sign) and traffic signals (red and green lights) in images. The goal is to recognize which of these objects appear in each image. As an example, the above image contains a pedestrian crossing sign and red traffic lights

Your 100,000 labeled images are taken using the front-facing camera of your car. This is also the distribution of data you care most about doing well on. You think you might be able to get a much larger dataset off the internet, that could be helpful for training even if the distribution of internet data is not the same.

You are just getting started on this project. What is the first thing you do? Assume each of the steps below would take about an equal amount of time (a few days).

Ans: Spend a few days training a basic model and see what mistakes it makes.

Note: As discussed in lecture, applied ML is a highly iterative process. If you train a basic model and carry out error analysis (see what mistakes it makes) it will help point you in more promising directions.

2. Your goal is to detect road signs (stop sign, pedestrian crossing sign, construction ahead sign) and traffic signals (red and green lights) in images. The goal is to recognize which of these objects appear in each image. You plan to use a deep neural network with ReLU units in the hidden layers.

For the output layer, a softmax activation would be a good choice for the output layer because this is a multi-task learning problem. True/False?

Ans: False

3. You are carrying out error analysis and counting up what errors the algorithm makes. Which of these datasets do you think you should manually go through and carefully examine, one image at a time?

Ans: 500 images on which the algorithm made a mistake

4. After working on the data for several weeks, your team ends up with the following data:

100,000 labeled images taken using the front-facing camera of your car.

Each image’s labels precisely indicate the presence of any specific road signs and traffic signals or combinations of them. For example, y(i) = ⎡⎣⎢⎢⎢⎢10010⎤⎦⎥⎥⎥⎥ means the image contains a stop sign and a red traffic light.

Because this is a multi-task learning problem, you need to have all your y(i) vectors fully labeled. If one example is equal to ⎡⎣⎢⎢⎢⎢0?11?⎤⎦⎥⎥⎥⎥ then the learning algorithm will not be able to use that example. True/False?

Ans: False

5. The distribution of data you care about contains images from your car’s front-facing camera; which comes from a different distribution than the images you were able to find and download off the internet. How should you split the dataset into train/dev/test sets?

Ans: Choose the training set to be the 900,000 images from the internet along with 80,000 images from your car’s front-facing camera. The 20,000 remaining images will be split equally in dev and test sets.

Note: As seen in lecture, it is important that your dev and test set have the closest possible distribution to “real”-data. It is also important for the training set to contain enough “real”-data to avoid having a data-mismatch problem.

6. Assume you’ve finally chosen the following split between of the data:

Dataset: Contains:         Error of the algorithm:

Training  940,000 images randomly picked from (900,000 internet images + 60,000 car’s front-facing camera images)  8.8%

Training-Dev  20,000 images randomly picked from (900,000 internet images + 60,000 car’s front-facing camera images)  9.1%

Dev 20,000 images from your car’s front-facing camera  14.3%

Test 20,000 images from the car’s front-facing camera    14.8%

You also know that human-level error on the road sign and traffic signals classification task is around 0.5%. Which of the following are True? (Check all that apply).

Ans:

You have a large data-mismatch problem because your model does a lot better on the training-dev set than on the dev set

You have a large avoidable-bias problem because your training error is quite a bit higher than the human-level error.

7. Based on table from the previous question, a friend thinks that the training data distribution is much easier than the dev/test distribution. What do you think?

Ans: There’s insufficient information to tell if your friend is right or wrong.

8. You decide to focus on the dev set and check by hand what are the errors due to. Here is a table summarizing your discoveries:

Overall dev set error     14.3%

Errors due to incorrectly labeled data 4.1%

Errors due to foggy pictures 8.0%

Errors due to rain drops stuck on your car’s front-facing camera    2.2%

Errors due to other causes   1.0%

In this table, 4.1%, 8.0%, etc.are a fraction of the total dev set (not just examples your algorithm mislabeled). I.e. about 8.0/14.3 = 56% of your errors are due to foggy pictures.

The results from this analysis implies that the team’s highest priority should be to bring more foggy pictures into the training set so as to address the 8.0% of errors in that category. True/False?

Ans: True, because it is the largest category of errors. As discussed in lecture, we should prioritize the largest category of error to avoid wasting the team’s time.

9. You can buy a specially designed windshield wiper that help wipe off some of the raindrops on the front-facing camera. Based on the table from the previous question, which of the following statements do you agree with?

Ans: 2.2% would be a reasonable estimate of the maximum amount this windshield wiper could improve performance.

10. You decide to use data augmentation to address foggy images. You find 1,000 pictures of fog off the internet, and “add” them to clean images to synthesize foggy days, like this:

Which of the following statements do you agree with? (Check all that apply.)

Ans: There is little risk of overfitting to the 1,000 pictures of fog so long as you are combing it with a much larger (>>1,000) of clean/non-foggy images.

11. After working further on the problem, you’ve decided to correct the incorrectly labeled data on the dev set. Which of these statements do you agree with? (Check all that apply).

Ans: You should also correct the incorrectly labeled data in the test set, so that the dev and test sets continue to come from the same distribution

12. So far your algorithm only recognizes red and green traffic lights. One of your colleagues in the startup is starting to work on recognizing a yellow traffic light. (Some countries call it an orange light rather than a yellow light; we’ll use the US convention of calling it yellow.) Images containing yellow lights are quite rare, and she doesn’t have enough data to build a good model. She hopes you can help her out using transfer learning.

What do you tell your colleague?

Ans: She should try using weights pre-trained on your dataset, and fine-tuning further with the yellow-light dataset.

13. Another colleague wants to use microphones placed outside the car to better hear if there’re other vehicles around you. For example, if there is a police vehicle behind you, you would be able to hear their siren. However, they don’t have much to train this audio system. How can you help?

Ans: Neither transfer learning nor multi-task learning seems promising.

Note: The problem he is trying to solve is quite different from yours. The different dataset structures make it probably impossible to use transfer learning or multi-task learning.

14. To recognize red and green lights, you have been using this approach:

(A) Input an image (x) to a neural network and have it directly learn a mapping to make a prediction as to whether there’s a red light and/or green light (y).

A teammate proposes a different, two-step approach:

(B) In this two-step approach, you would first (i) detect the traffic light in the image (if any), then (ii) determine the color of the illuminated lamp in the traffic light.

Between these two, Approach B is more of an end-to-end approach because it has distinct steps for the input end and the output end. True/False?

Ans: False

15. Approach A (in the question above) tends to be more promising than approach B if you have a ________ (fill in the blank).

Ans: Large training set

0
0 收藏

0 评论
0 收藏
0