---
title: "Data reduction strategies"
subject: "Data Mining"
chapter: 2
key: "semester_7/data_mining:data-reduction"
source: https://www.bctengineer.com/q/semester_7/data_mining/data-reduction
site: BCT Engineering
---

# Data reduction strategies

Data Mining · Chapter 2: Data Pre-processing

- Asked in: year not given
- Video: [Data Reduction Techniques Part-1 Explained with Example in Hindi](https://www.youtube.com/watch?v=0l3tzQI_ApM) — 5 Minutes Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/data_mining/data-reduction
- Whole chapter: https://www.bctengineer.com/md/semester_7/data_mining/chapter-2

## Question

Explain data reduction with strategies.

## Short answer

Data reduction obtains a smaller dataset that yields similar analytical results. Strategies include dimensionality reduction (feature selection/extraction), numerosity reduction (parametric models, clustering, sampling), data compression, and data cube aggregation.

## Answer

**Data reduction** obtains a much smaller representation of the data set that produces the same (or almost the same) analytical results. It cuts storage and mining time on huge data.

#### Strategies
1. **Dimensionality reduction**: reduce the number of attributes.
   - Feature selection (remove irrelevant/redundant attributes)
   - Feature extraction such as PCA and wavelet transform
2. **Numerosity reduction**: replace the data by a smaller representation.
   - *Parametric*: fit a model (regression, log-linear model) and store only the parameters.
   - *Non-parametric*: histograms, clustering (store the cluster centres), sampling, data cube aggregation.
3. **Data compression**: lossless (e.g. run-length, Huffman) or lossy (e.g. wavelets, JPEG) encoding of the data.
4. **Data cube aggregation**: aggregate data to a higher level, e.g. quarterly sales instead of daily sales.
5. **Discretization and concept hierarchy generation**: replace numeric values by intervals or higher-level concepts.
6. **Sampling**: use a representative subset of tuples (see sampling methods).

#### Example
Daily sales for 5 years (1825 values) can be aggregated to 20 quarterly totals; a 100-attribute table can be reduced to its 10 principal components.

The reduction itself must be cheap compared with the mining time it saves.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is data reduction?
   A. A much smaller representation giving the same or almost the same analytical results
   B. Increasing attributes
   C. Removing all noise
   D. Encrypting data
   Answer: A. It cuts storage and mining time. (Key point: Definition)

2. What is parametric numerosity reduction?
   A. Store cluster centres
   B. Fit a model such as regression and store only its parameters
   C. Draw a sample
   D. Build a histogram
   Answer: B. Histograms, clustering and sampling are non-parametric. (Key point: Parametric numerosity)

3. Which is an example of data cube aggregation?
   A. Filling missing values
   B. Z-score normalisation
   C. Daily sales aggregated to quarterly totals
   D. Removing duplicates
   Answer: C. 5 years of daily values (1825) become 20 quarterly totals. (Key point: Cube aggregation)

4. Which pairing of compression types is correct?
   A. Lossless: JPEG; lossy: Huffman
   B. Both lossless: wavelets, Huffman
   C. Both lossy: run-length, Huffman
   D. Lossless: run-length, Huffman; lossy: wavelets, JPEG
   Answer: D. Lossless codes can fully restore data, lossy cannot. (Key point: Compression)

## Related in Data Mining

- [Dimensionality reduction techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Adimensionality-reduction) — Data Mining, chapter 2

## Related in other subjects

- [Switching techniques in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-techniques-data-comm) — Telecommunications, chapter 8, asked 3×
- [Tensile test data of a steel bar](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Atensile-test-numerical-steel-bar) — Material Science, chapter 3