Definition
PageRank is an algorithm developed by Google's founders that ranks web pages based on the number and quality of links pointing to them.
Key Point
PageRank Core Concept:
A page is important if other important pages link to it.
Key Factors:
• Number of incoming links (more = better)
• Quality of linking pages (link from BBC > link from unknown blog)
• Number of outgoing links on linking page (fewer = each link worth more)
• Damping factor (typically 0.85) - holds a score down, on the idea that a user will only click through a certain number of links before stopping
A page is important if other important pages link to it.
Key Factors:
• Number of incoming links (more = better)
• Quality of linking pages (link from BBC > link from unknown blog)
• Number of outgoing links on linking page (fewer = each link worth more)
• Damping factor (typically 0.85) - holds a score down, on the idea that a user will only click through a certain number of links before stopping
Worked Example
Worked Example: Understanding PageRank
Scenario: Three pages: A, B, and C
Links:
• Page A links to B and C
• Page B links to C
• Page C links to A
Analysis:
• Page C has 2 incoming links (from A and B) - highest
• Page A has 1 incoming link (from C)
• Page B has 1 incoming link (from A)
But quality matters too:
If Page A is a major news site with high PageRank,
its link to B is worth more than B's link to C.
Result: Pages with more high-quality incoming links rank higher.
Scenario: Three pages: A, B, and C
Links:
• Page A links to B and C
• Page B links to C
• Page C links to A
Analysis:
• Page C has 2 incoming links (from A and B) - highest
• Page A has 1 incoming link (from C)
• Page B has 1 incoming link (from A)
But quality matters too:
If Page A is a major news site with high PageRank,
its link to B is worth more than B's link to C.
Result: Pages with more high-quality incoming links rank higher.
Exam Tip
Other Ranking Factors:
• Keyword relevance and placement
• Page loading speed
• Mobile-friendliness
• Content freshness
• User engagement (click-through rate, time on page)
• Secure connection (HTTPS)
• Keyword relevance and placement
• Page loading speed
• Mobile-friendliness
• Content freshness
• User engagement (click-through rate, time on page)
• Secure connection (HTTPS)
The PageRank Formula
PageRank treats the web as a directed graph: each page is a node and each hyperlink is an edge that points from the linking page to the page it links to. The score of a page is worked out from the scores of the pages that link to it.
text
PR(A) = (1 - d) + d × ( PR(T1)/C(T1) + PR(T2)/C(T2) + ... + PR(Tn)/C(Tn) )Key Point
What the terms mean:
• PR(A) is the PageRank of page A, the page being scored
• T1 to Tn are the pages that link to page A
• PR(T) is the PageRank of a linking page: a link from a highly ranked page is worth more
• C(T) is the number of outbound links on that linking page: its PageRank is shared between all the pages it links to
• d is the damping factor, a value between 0 and 1, usually about 0.85
• PR(A) is the PageRank of page A, the page being scored
• T1 to Tn are the pages that link to page A
• PR(T) is the PageRank of a linking page: a link from a highly ranked page is worth more
• C(T) is the number of outbound links on that linking page: its PageRank is shared between all the pages it links to
• d is the damping factor, a value between 0 and 1, usually about 0.85
Definition
The damping factor is a value between 0 and 1 that allows for the fact that a user does not go on following links for ever. With d = 0.85, the model assumes that at each page there is an 85% chance the user follows another link and a 15% chance that they stop.
Worked Example
Worked Example: Using the Formula
Two pages link to page A, and d = 0.85:
• Page B has a PageRank of 2 and 4 outbound links
• Page C has a PageRank of 3 and 2 outbound links
PR(A) = (1 - 0.85) + 0.85 × (2/4 + 3/2)
PR(A) = 0.15 + 0.85 × 2
PR(A) = 1.85
The link from C adds more than the link from B, because C has a higher PageRank and shares it between fewer links.
Two pages link to page A, and d = 0.85:
• Page B has a PageRank of 2 and 4 outbound links
• Page C has a PageRank of 3 and 2 outbound links
PR(A) = (1 - 0.85) + 0.85 × (2/4 + 3/2)
PR(A) = 0.15 + 0.85 × 2
PR(A) = 1.85
The link from C adds more than the link from B, because C has a higher PageRank and shares it between fewer links.
Every page's score depends on the scores of other pages, so the calculation is repeated for all pages until the values settle.