Why is human psychology and perception an important factor in methods of Lossy compression?

This is because in lossy compressions, very little of the information from a file is still there. Without the deception of how the human eye sees the difference of information, the file would be very different from the original. If we allow our mind to do so would be able to the errors in a lossy compression.

Which method offers the greatest compression ratio?

The method that offers the greatest compression ratio is lossy compression. The compression ratio of lossy video codecs are generally better than audio and still-image.
The video can be compressed at a 100:1 ratio 
Audio can often be compressed at a 10:1 
Still images are also often compressed at a 10:1 ratio.
The compression rate of lossy compression is about 5-6% while the rate of lossless compression ranges from 50 60%.

What is the difference between Lossy and Lossless data compression?

Lossy compression technologies attempt to eliminate redundant or unnecessary information. Most video compression technologies, such as MPEG, use a lossy technique.

lossless compression refers to data compression techniques in which no data is lost. For most types of data, lossless compression techniques can reduce the space needed by only about 50%

Why is data compression an important technique for the online world?

Data compression is very important for the online world. When you visit a website, all the picture files that are downloaded are all compressed. This also includes videos when online.
Files that are fairly can be compressed so that they are smaller in size. making it more efficient when downloading or uploading.

What is Data Compression?

Data compression is basically storing data in a format that requires less space than usual. It is the process of encoding information such as videos, images and audios in fewer bits than its original. It reduces size as well as quality of data. 

What is a data compression ratio?

You can set the compression ratio for color scanned data. A high compression ratio reduces the data size, but results in low quality images. On the contrary, a low compression ratio increases the data size, but results in high quality images.