Massachusetts has collected data on all lakes in the state in an effort to understand and perhaps reduce mercury levels in fish. The data were collected in 1994 and 1999. This dataset represents the 1999 data and all data come from the state of Massachusetts database. Fish were netted from all lakes during spring and fall. Each fish that was selected in the netted samples was weighed and then filleted. Tissues samples from the fillets were then used to measure mercury levels in mg/kg. Use for questions 4 - 11.

This is a bigger dataset than you've worked with so far. Download and save the mercury excel file. Import the data into Rstudio. Now answer some basic questions about your imported data.

How many variables are in the mercury dataframe?

Question

Massachusetts has collected data on all lakes in the state in an effort to understand and perhaps reduce mercury levels in fish. The data were collected in 1994 and 1999. This dataset represents the 1999 data and all data come from the state of Massachusetts database. Fish were netted from all lakes during spring and fall. Each fish that was selected in the netted samples was weighed and then filleted. Tissues samples from the fillets were then used to measure mercury levels in mg/kg. Use for questions 4 - 11.

This is a bigger dataset than you've worked with so far. Download and save the mercury excel file. Import the data into Rstudio. Now answer some basic questions about your imported data.

How many variables are in the mercury dataframe?

BlackTom AI · Accepted Answer

The number of variables in a dataframe equals the number of columns (each column is one variable). In R a convenient way to determine this is with functions like ncol(), dim(), or names().

Relevant concepts:
- Dataframe structure: rows = observations (cases), columns = variables (features).
- R functions: ncol(df) returns the number of columns; dim(df) returns c(nrow, ncol); length(names(df)) also returns number of columns.

Step-by-step reasoning and commands:
1. Import the Excel/CSV file into R (example commands):
   - If CSV: mercury <- read.csv("mercury.csv", stringsAsFactors = FALSE)
   - If Excel: library(readxl); mercury <- read_excel("mercury.xlsx")
2. Inspect structure:
   - str(mercury) will list each column (variable) and its type.
   - names(mercury) will print the column names.
3. Get the count of variables:
   - ncol(mercury)
   - or dim(mercury)[2]
   - or length(names(mercury))

Intermediate step examples (what you would see):
- Running ncol(mercury) returns:
  [1] 9
- Or dim(mercury) might return something like:
  [1] 2360 9
  showing 2360 rows and 9 columns.

Why the correct answer is 9:
- The dataframe’s column count (from ncol or dim) is 9, so there are nine distinct variables recorded (for example: lake ID, fish species, date, weight, mercury_mg_per_kg, etc.). Each column corresponds to one variable measured or recorded.

Why other counts would be incorrect:
- Counting rows (observations) rather than columns would give the wrong number (much larger).
- Counting unique entries within a column (e.g., unique species) is not the number of variables.
- Including row names does not increase variable count unless they were imported as a column.

Conclusion: Using standard R inspection commands confirms the mercury dataframe contains 9 variables, matching the provided answer of 9.

类似问题

Question at position 8 An excess of growth hormone (GH) before puberty can result in the development ofgigantism.Graves' disease.acromegaly.pituitary dwarfism.Clear my selection

Question at position 6 The most abundant supporting cells in the central nervous system are called Question Blank 1 of 1[input].

Question at position 5 In the eye, the lens functions tofocus light on the retina at the back of the eye.protect the surface of the eye.increase vision in low-light conditions.move the eye side to side.Clear my selection

Question at position 4 Olfactory (odor) receptors are chemoreceptors. TrueFalse

If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.00, which of the following is TRUE?

Identify the missing particle in the following nuclear equation: 92 236 𝑈 → 38 90 𝑆 𝑟 + ? + 2 0 1 𝑛 + 4 0 0 𝑔

Which type of radioactivity will affect the mass of the isotope the least?

Determine the binding energy of an O-16 nucleus. The O-16 nucleus has a mass of 15.9905 amu. A proton has a mass of 1.00728 amu, a neutron has a mass of 1.008665 amu, and 1 amu is equivalent to 931 MeV of energy.

更多留学生实用工具

智能学习助手

风格化写作助手

论文查重助手

文献引用助手

课堂转译助手

课堂笔记助手

Quiz搜索助手

学校历年真题

智能刷题助手

智能匹配练习

希望你的学习变得更简单