The patents issued by the program have an average of 4.3 US classifications. From a broad subject matter standpoint, the most complex Green Tech program patents were primarily chemical, materials engineering, and electrical/electronics subject matter.

Analyzing Patent Complexity

Patents granted under the program were assessed to examine the complexity of the content of the inventions. One of the ways to analyze patent complexity is to look at the patent classifications assigned by the USPTO. Looking at classifications gives you a fingerprint of what an invention is and all of its important parts. A patent's classification data will tell you scientific or technical content of an invention according to USPTO, which group of examiners evaluated the patent and the other technology grouped in the same area; and how many different subjects are covered by the invention. Classifications can provide an indication that patents in one domain are converging with patents in another. For example, the first time an invention with a miniaturized camera was classified with a personal electronic device (smartphone) or US patent 7,531,007, a smoke detecting mobile device — the presence of a classification for a sensor capable of detecting chemicals found in smoke from a fire is present in cellphone.

The current US Patent Classfication (USPC) system has over 150,000 classifications. The new and emerging Cooperative Patent Classification system released in January 2013 promises to have over 200,000 classifications. Patent classification systems allow for very granular identification of the content of an invention. The United States Patent Classification (USPC) system is used to organize US patent documents (patent applications, patents, prior art technical and scientific material) into relatively small collections based on a common subject matter. USPTO defines classes — major boundaries between one technology and another — and subclasses which delineate processes, structural features, and functional features of the subject matter within a class. The class is the high level topic. The subclass is the breakdown of all the different things that have been invented in that class. Patents are classified at the subclass level. The goal is to assign the patent to the subclass that most accurately describes the content. Subclasses are the smallest division of subject matter that USPTO uses to organize subject matter.

A classification number looks like this 705/26.1 — class/subclass. The USPC Class is 705: DATA PROCESSING: FINANCIAL, BUSINESS PRACTICE, MANAGEMENT, OR COST/PRICE DETERMINATION. The subclass is 26.1: Electronic Shopping. So patents classified as 705/26.1 are about business methods for electronic shopping.

A fingerprint

A patent's classifications are a fingerprint of the technology in the invention. The classifications on a patent identify all of the relevent subject matter (unique technology, science, the parts of the invention) covered in a patent. Every patent has at least one US classification, the original, or OR, which is the first USPC classification listed on a patent. Additional cross-reference classifications (those following the OR classification on the patent) are assigned based on a set of classification rules that apply to a patent's claims and other disclosed material (e.g., Specification, Drawings). Mandatory classifications pertaining to the claims are required, and discretionary classifications pertaining to the remainder of the disclosed information in a patent may be assigned at the discretion of the examiner to aid in prior art searches. The mandatory classifications provide very useful information on the complexity of a patent, but USPTO chooses to create a giant blob of patent classifications that doesn't distinguish between mandatory and discretionary classifications. (Patent cognoscenti would love to have the breakdown. If you would like one, let us know.)

(To learn more about classification see Classification Primer)

Summary Statistics

Inferences about the complexity of green technology inventions were drawn by the number of classifications assigned to it by USPTO. Here is a look at the complexity of the green technology patents granted under the USPTO Green Tech Pilot Program.

The number of classifications (OR + cross-references) on the 836 patents granted through February 14, 2012 ranges from 1-20, with a mean of 4.3, a median of 3 (i.e., 50% of the patents have 1-3 total classifications, and 50% have 4 or more), and a standard deviation of 3.13.

The frequency distribution of total number of classifications is shown in the following chart.

Classification Count

Frequency of Number of Classifications

There is a strong unimodal distribution centered on two USPC classifications, which represented 192 patents. Seventy-eight (78) percent of the patents have five or fewer classifications.

Index View

As expected, there are differences in the average complexity (i.e., the average number of USPC classifications per patent) across the Indexes. Patents in the Air and Water Indexes are considerably more complex than those in the other four groups of patents, with Transportation Index patents being the least complex in terms of number of classifications assigned to the patents.

Patent Complexity by patentECO™ Index
patentECO™ IndexAverage # of Classifications

Category View

The patent categories whose complexity is greater than the average (i.e., 4.3) of the entire data set are presented below.

Water generation (in this case, one patent pertaining to solar desalination) is the most complex category for the patents issued by the USPTO program. This is followed by photovoltaic-produced energy (1 patent), air emissions controls (19 patents), wastewater treatment (16 patents), industrial disaster response (1 patent, a spill avoidance system) and biofuels (14 patents). Categories with larger numbers of issued patents tend to cluster toward the least complex (i.e., lower average number of classifications) end of the range shown here.

Patent Complexity by patentECO™ Category
patentECO™ CategoryAverage # of Classifications# of Patents
Water: Generation141
Energy: Photovoltaics122
Air: Emissions controls8.7419
Air: Carbon capture or sequestration82
Water: Wastewater treatment7.5616
Industry: Disaster71
Energy: Biofuel6.4416
Energy: Fossil fuel6.119
Agriculture: Fertilizer alternative61
Transportation: Roadway61
Transportation: Vehicle66
Industry: Materials5.8513
Industry: Production5.710
Industry: Recycling5.336
Energy: Lighting5.0788
Energy: Energy efficiency4.5958
Industry: Chemical engineering4.5728
Energy: Fuel cell4.3719

Technology View

The most complex technologies are those with patent classifications equal to or greater than twice the average (i.e., 2 X 4.3 = 8.6). This measure identifies 11 technologies that received patents under the Green Tech Pilot Program.

Patent Complexity by patentECO™ Technology
patentECO™ Index, Category, & TechnologyAverage # of Classifications# of Patents
Industry : Recycling : Plastic/rubber171
Water : Wastewater treatment : Produced water161
Water : Generation : Desalination 14 1
Water : Wastewater treatment : Filtration 14 2
Air : Carbon capture or sequestration : Industrial carbon capture 121
Water : Wastewater treatment : Bioretention101
Energy : Energy efficiency : Waste gas treatment9.52
Water : Wastewater treatment : Dewatering9.52
Water : Wastewater treatment : Sand filter 9 1
Air : Emissions controls : Waste gas treatment8.7419
Energy : Fossil fuel : Petroleum8.673

The most complex technology (Plastic/Rubber) found in patents issued by the USPTO Green Tech program thus far is the patent for tire recycling, US Patent 7,959,890, "Method of Reclaiming Carbonaceous Materials from Scrap Tires and Products Derived Therefrom" in the Industry Index. The second most complex technology (Produced water) is a wastewater treatment patent using dry pond evaporation. The remaining technologies shown here are the subject of 1-3 patents each, with the exception of Waste gas treatment in the Air Index, which is the subject of 19 patents.

Assignee View

The chart that follows presents information on the assignees awarded patents with nine (9) or more US patent classifications assigned.

Patent Complexity by Assignee
AssigneeAverage # of Classifications# of Patents
RIPP Resource Recovery Corporation171
Reform Water, LLC161
San Diego Gas & Electric Company161
King Fahd University of Petroleum and Minerals 14 1
Panasonic Corporation 13 1
Accenture Global Services GmbH 12 1
American Pacific Corporation 12 1
Rutgers, The State University of New Jersey 12 1
Ibex Incorporated 11 1
Membrane Technology & Research, Inc 10.5 4
Airgard, Inc. 10 1
Kior Inc. 10 6
PetroResOne, Inc. 10 1
Poet Research, Inc. 10 2
ALSTOM Technology Ltd 9.5 2
Avello Bioenergy, Inc. 9 1
EcoFactor, Inc. 9 1
EnerTech Environmental, Inc. 9 2
Synthetic Genomics, Inc. 9 1
The Curators of the University of Missouri 9 1

RIPP Resource Recovery Corp. is the assignee with the patent with the most USPC classifications — 17 classifications. Reform Water, LLC and San Diego Gas & Electric each have Green Tech program patent portfolios averaging 16 classifications for the single patents granted under the program. Kior Inc. owns six of the Green Tech patents, averaging 10 classifications, and has the distinction of owning the most complex Green Tech patent granted thus far, based on total number of USPC classifications, with 20 classifications. That patent is a biofuel synthesis invention found in the patentECO™ Energy Index.


The subclass placement of the Original Classification (OR) on the patent, the first classification that appears on a patent, provides insight into the complexity of a patent. The USPC uses a general hierarchy that places the most complex subject matter higher in the class schedule. Subclasses that are indented the most represent the most complex subclasses of a US patent class. To see which patents are the most complex we looked at the classification based on their hierarcy in the class itself. The following chart illustrates the ranking of patent complexity by OR classes for patents with an average number of classifications greater than 8.6.

Taking each of the Green Technology Pilot Program patents singly, without regard to average numbers of classifications, one patent was issued with 20 classifications (to Kior Inc., as noted above). This was a biofuel synthesis invention placed in USPC class 585, for chemistry of hydrocarbon compounds. The most complex patent in terms of the average number of classifications is found within class 202, which covers distilling apparatus inventions.

Patent Complexity by Original Class (OR)
Average # of Classifications# of Patents USPC Class & Title
1414202: Distillation: Apparatus
12 1169: Fire extinguishers
10.5296: Gas separation: Apparatus
103159: Concentrating evaporators
104585: Chemistry of hydrocarbon compounds
9995: Gas separation: Processes
93201: Distillation: Processes, thermolytic
9 4 208: Mineral oils: Processes and products

Tech Center View

Finally, how does Green Tech patent complexity vary by USPTO Tech Center? Green Tech patents examined in Technology Center (TC) 1700 had the greatest average complexity, which is not surprising given the highly complex chemical and materials engineering patents that TC 1700 is responsible for. This is followed by TC 2100, computer architecture, software, and information security. The third-ranked TC was TC 2600, responsible for communications technology. Slightly behind those three was TC 2400, responsible for computer networks, multiplex communication, video distribution, and security. Thus, from a broad subject matter standpoint, the most complex Green Tech program patents were primarily chemical, materials engineering, and electrical/electronics subject matter.

Patent Complexity by USPTO Tech Center
Average # of Classifications# of Patents USPTO Tech Center
5.71951700: Chemical & Materials Engineering
5.5382100: Computer Architecture, Software, and Information Security
5.4372600: Communications
512400: Computer Networks, Multiplex communication, Video Distribution, and Security
3.91551600: Biotechnology & Organic Chemistry
3.832173700: Mechanical Engineering, Manufacturing, Products
3.792882800: Semiconductors, Electrical and Optical Systems and Components
2.74353600: Transportation, Construction, Electronic Commerce, Agriculture, National Security and License & Review


The complexity of patents issued by the USPTO Green Technology Pilot Program may be assessed based on the average number of classifications applied to various groupings of patents. Limitations on the number of independent claims, and total number of claims, for program eligibility make standard claims analysis measures (e.g., number of claims, length of claims, independent vs. dependent claims) meaningless. The 836 patents issued by the program have an average of 4.3 US classifications, and range from 1-20. The largest group of patents (192) have two (2) classifications. Seventy-eight (78) percent of the patents have five or fewer classifications.

Average number of claims

Patents in the patentECO™ Air Index have the greatest complexity as measured by the average number of classifications — 8.04. Water Index inventions are only slightly less complex (7.56 average US classifications). There is a significant decline in average complexity between these two groups and each of the others for which patents were issued by the program (Industry, Energy, Agriculture, Transportation). The two most complex categories of inventions were water generation (one patent with 14 classifications) and energy from photovoltaics (2 patents that averaged 12 classifications). A significant decline is observed for the other categories. At the more granular technology level, a single plastic/rubber recycling patent within the Industry Index was the most complex with 17 classifications. Slightly less complex (16 classifications) was a single wastewater treatment patent for produced water. Kior Inc. had the distinction of being awarded the most complex patent with 20 classifications. Four USPTO Tech Centers (1700, 2100, 2600, 2400) granted significantly more complex patents than the other centers. From a broad subject matter standpoint, the most complex Green Tech program patents were primarily chemical, materials engineering, and electrical/electronics subject matter.

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